sdk(java): Reject null in withContext to match the other SDKs

Java accepted null as a "clear" operation while .NET rejects it and the
other SDKs disallow it at the type level. Since null is Java's default,
an uninitialized variable would have silently dropped the context --
producing exactly the unattributed telemetry this feature removes.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 79152db2-4cc7-4777-983a-655fae5b68c9

Author: aymenfurter

java/src/main/java/com/github/copilot/rpc/PermissionRequestResult.java

@@ -5,6 +5,7 @@
 package com.github.copilot.rpc;
 
 import java.util.List;
+import java.util.Objects;
 
 import com.fasterxml.jackson.annotation.JsonIgnore;
 import com.fasterxml.jackson.annotation.JsonInclude;
@@ -202,12 +203,14 @@ public PermissionDecisionContext getDecisionContext() {
      * sibling of {@code result}.
      *
      * @param context
-     *            the decision context, or {@code null} to clear it
+     *            the decision context; must not be {@code null}
      * @return this result for method chaining
+     * @throws NullPointerException
+     *             if {@code context} is {@code null}
      * @since 1.3.0
      */
     public PermissionRequestResult withContext(PermissionDecisionContext context) {
-        this.decisionContext = context;
+        this.decisionContext = Objects.requireNonNull(context, "context must not be null");
         return this;
     }
 }

java/src/test/java/com/github/copilot/rpc/PermissionRequestResultDecisionContextTest.java

@@ -8,6 +8,7 @@
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import com.fasterxml.jackson.databind.JsonNode;
@@ -85,4 +86,12 @@ void serializingResultWithContextDoesNotEmitContextInsideResult() throws Excepti
                 "@JsonIgnore must keep decisionContext out of the serialized result");
         assertEquals(PermissionRequestResultKind.APPROVED.getValue(), resultJson.get("kind").asText());
     }
+
+    @Test
+    void withContextRejectsNull() {
+        var result = PermissionRequestResult.approveOnce();
+
+        assertThrows(NullPointerException.class, () -> result.withContext(null),
+                "withContext must reject null rather than silently dropping the context");
+    }
 }