feat(gax): add ResumableUploadFutureImpl state machine and ResumableUploadClient SPI

- Add ResumableUploadFutureImpl stateful per-request state machine.
- Add ResumableUploadClient transport SPI interface and value objects (ResumableUploadSession, ChunkUploadRequest, ChunkUploadResponse).
- Update ResumableUploadCallable to instantiate and start ResumableUploadFutureImpl.

Author: blakeli0

sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/rpc/ChunkUploadRequest.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google LLC nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package com.google.api.gax.rpc;
+
+import com.google.api.core.BetaApi;
+import com.google.common.base.Preconditions;
+
+/** Value object representing a chunk upload request. */
+@BetaApi
+public class ChunkUploadRequest {
+
+  private final String uploadUrl;
+  private final byte[] data;
+  private final long offset;
+  private final long totalSize;
+  private final boolean isLast;
+
+  public ChunkUploadRequest(
+      String uploadUrl, byte[] data, long offset, long totalSize, boolean isLast) {
+    this.uploadUrl = Preconditions.checkNotNull(uploadUrl);
+    this.data = Preconditions.checkNotNull(data);
+    this.offset = offset;
+    this.totalSize = totalSize;
+    this.isLast = isLast;
+  }
+
+  public String getUploadUrl() {
+    return uploadUrl;
+  }
+
+  public byte[] getData() {
+    return data;
+  }
+
+  public long getOffset() {
+    return offset;
+  }
+
+  public long getTotalSize() {
+    return totalSize;
+  }
+
+  public boolean isLast() {
+    return isLast;
+  }
+}

sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/rpc/ChunkUploadResponse.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google LLC nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package com.google.api.gax.rpc;
+
+import com.google.api.core.BetaApi;
+import javax.annotation.Nullable;
+
+/** Value object representing a chunk upload response. */
+@BetaApi
+public class ChunkUploadResponse {
+
+  private final long committedOffset;
+  @Nullable private final Object responseBody;
+
+  public ChunkUploadResponse(long committedOffset, @Nullable Object responseBody) {
+    this.committedOffset = committedOffset;
+    this.responseBody = responseBody;
+  }
+
+  public long getCommittedOffset() {
+    return committedOffset;
+  }
+
+  @Nullable
+  public Object getResponseBody() {
+    return responseBody;
+  }
+}

sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/rpc/ResumableUploadClient.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google LLC nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package com.google.api.gax.rpc;
+
+import com.google.api.core.BetaApi;
+
+/**
+ * Low-level transport Service Provider Interface (SPI) for executing individual Scotty resumable
+ * upload operations (session initiation, chunk upload, and status querying).
+ */
+@BetaApi
+public interface ResumableUploadClient {
+
+  /** Returns a UnaryCallable to initiate a new resumable upload session. */
+  <RequestT> UnaryCallable<RequestT, ResumableUploadSession> startUploadCallable();
+
+  /** Returns a UnaryCallable to upload a payload byte chunk to an active session URL. */
+  UnaryCallable<ChunkUploadRequest, ChunkUploadResponse> uploadChunkCallable();
+
+  /** Returns a UnaryCallable to query the server's committed byte offset for a session URL. */
+  UnaryCallable<String, Long> queryStatusCallable();
+}