fix: TaskContextError for cross-task usage (issue #576)

[gspeter-max]

Implementation: Fix for Issue #576 - Cross-Task Detection

Implements suggestion #2 from the issue: detect cross-task usage and raise a clear error instead of silently hanging.

What Was Changed

1. New Exception: TaskContextError

• Added TaskContextError exception class to _errors.py
• Stores connect_task_id and current_task_id for debugging
• Includes helpful error message with guidance

2. Task Tracking in Query Class

• Added _owner_task field to track which task owns the Query instance
• Captures task ID in start() using anyio.get_current_task()
• Validates task context in receive_messages()

3. Updated Documentation

• Updated caveat in client.py to reference the new error
• Added FastAPI example showing correct usage patterns

4. Comprehensive Tests

• tests/test_task_context.py: 6 new tests covering:
  • Same-task usage (works)
  • Cross-task usage (raises TaskContextError)
  • Multiple clients in different tasks (all work)
  • Error attributes and messages

Example of the New Behavior

Before (silent hang):

client = ClaudeSDKClient()
await client.connect()  # Task A

async def handle_request():
    async for msg in client.receive_messages():  # Task B
        yield msg
    # ← Hangs here forever with no error

After (clear error):

client = ClaudeSDKClient()
await client.connect()  # Task A

async def handle_request():
    async for msg in client.receive_messages():  # Task B
        yield msg
    # ← Raises: TaskContextError: ClaudeSDKClient cannot be used across 
    #    different async tasks. Each async task must create its own client 
    #    instance. See TaskContextError documentation for details.
    #    (connect task: 12345, current task: 67890)

Correct Usage for FastAPI

@app.post("/query")
async def query_endpoint(prompt: str):
    # Create client per-request, in the request handler's task
    async with ClaudeSDKClient() as client:
        await client.query(prompt)
        async for msg in client.receive_messages():
            yield msg

See examples/fastapi_example.py for complete working example.

Note on Test Changes

One existing test (test_concurrent_send_receive in test_streaming_client.py) was removed because it tested the exact cross-task pattern we're now preventing. This test:

• Used asyncio.create_task() to call receive_messages() from a different task
• Only passed with mocked transports (false confidence)
• Would hang with real transports (the bug we're fixing)
• Violated the documented caveat about cross-task usage

Cross-task usage has never been supported (documented since v0.0.20), and this test was giving false confidence about a buggy pattern.

Verification

• ✅ All 161 tests pass
• ✅ Type checking passes (mypy)
• ✅ Linting passes (ruff)
• ✅ New tests demonstrate the error is raised correctly
• ✅ FastAPI example added

Breaking Changes

None. This only adds validation that was missing. Existing valid single-task usage is unchanged.

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Ready for review! Feedback welcome on the error message wording, documentation, or approach.

Fixes #576