fix(python): serialize concurrent client startup

[xianjianlf2]

Concurrent calls to Python CopilotClient.start() can both observe a disconnected/connecting client, launch separate runtimes, and overwrite the shared connection. Concurrent session creation can take this path through auto-start as well. Serialize the state check and complete startup sequence with a per-client asyncio lock, so waiting callers reuse the established connection.

Related to #2560. That report and #2561 cover Node.js; this PR reproduces and fixes the same race in the Python implementation only. It does not close the Node.js issue.

Regression tests pause startup independently during process creation, connection setup, and protocol verification. All three reproduce duplicate initialization before the fix. Additional tests cover retry after startup failure and cancellation of a waiting caller without cancelling the active startup. The latter also reproduced duplicate process creation before the fix.

Validation:

• Before: 4 regression failures, 1 pass.
• After: 37 tests passed across test_client_start.py, test_jsonrpc.py, test_rpc_timeout.py, test_session.py, and test_github_token_provider.py.
• uv run ruff check, uv run ruff format --check ., and uv run ty check copilot passed.
• Live CLI/E2E tests were not run; startup phases are mocked to control scheduling deterministically.

AI-assisted implementation and validation.