fix: support mcp 2.x, which breaks the client on a fresh install (#50)

pyproject pins "mcp>=1.9.0" with no upper bound, so any new install
resolves mcp 2.0.0 and every XpozClient() raises immediately:

  ExceptionGroup: unhandled errors in a TaskGroup
    ValueError: not enough values to unpack (expected 3, got 2)

Two breaking changes in mcp 2.0:

- streamable_http_client yields 2 values instead of 3, so the
  "read, write, _ = streams" unpack fails.
- CallToolResult.isError was renamed to is_error.

Both are now handled without pinning, so the SDK works on 1.x and 2.x.

This is the same ValueError behind the errors seen across the live-API
test suites; tests/test_instagram.py goes from erroring to 13/13 passing
against the live API on mcp 2.0.0.


Claude-Session: https://claude.ai/code/session_011ekeWsNRndQwdgUE1SHqfe

Co-authored-by: claude-flow <ruv@ruv.net>

Author: b88fun

src/xpoz/_mcp/_transport.py

@@ -28,8 +28,19 @@ def _resolve_user_agent(override: str | None) -> str:
     return override
 
 
+def _unpack_streams(streams: Any) -> tuple[Any, Any]:
+    read_stream, write_stream = streams[0], streams[1]
+    return read_stream, write_stream
+
+
+def _is_error_result(result: Any) -> bool:
+    if hasattr(result, "is_error"):
+        return bool(result.is_error)
+    return bool(result.isError)
+
+
 def _parse_tool_result(tool_name: str, result: Any) -> dict[str, Any]:
-    if result.isError:
+    if _is_error_result(result):
         error_text = ""
         for block in result.content:
             if hasattr(block, "text"):
@@ -71,7 +82,7 @@ async def connect(self) -> None:
         streams = await ctx.__aenter__()
         self._context_stack.append(ctx)
 
-        read_stream, write_stream, _ = streams
+        read_stream, write_stream = _unpack_streams(streams)
         session_ctx = ClientSession(read_stream, write_stream)
         self._session = await session_ctx.__aenter__()
         self._context_stack.append(session_ctx)
@@ -139,7 +150,7 @@ async def _lifecycle(
                 timeout=httpx.Timeout(30, read=None),
             )
             async with streamable_http_client(self._server_url, http_client=http_client) as streams:
-                read_stream, write_stream, _ = streams
+                read_stream, write_stream = _unpack_streams(streams)
                 async with ClientSession(read_stream, write_stream) as session:
                     await session.initialize()
                     self._session = session