fix(mcp): serialize manager lifecycle operations

[abhinavkr26104]

This pull request resolves #4334 by serializing the public MCPServerManager lifecycle operations.

When connect_all(), reconnect(), or cleanup_all() overlapped in parallel mode, multiple cleanup commands could be submitted to a per-server worker. The worker exited after the first cleanup, leaving later command futures unresolved and callers hanging.

This change:

• Adds an internal lifecycle lock around the three public lifecycle entry points.
• Keeps nested internal cleanup unlocked so full reconnect does not deadlock.
• Preserves existing per-server worker task affinity and timeout behavior.
• Adds a deterministic regression test for overlapping parallel cleanup calls.

Verification:

• git diff --check: passed.
• Python AST parsing: passed.
• Repository verification could not run because this environment does not have make, uv, or pytest installed.

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[abhinavkr26104]

Addressed in commit 4db5a8df.

In parallel mode, _cleanup_server() now returns when the manager has neither a worker nor a connected-server marker for that server. This makes an overlapping cleanup that acquires the lifecycle lock after the first cleanup a no-op, instead of falling through to direct server.cleanup() in the caller task.

The existing regression uses a task-bound server and asserts that overlapping cleanup performs exactly one cleanup, so it covers the reported ownership issue. Static AST parsing and git diff --check pass; the full repository stack remains unavailable because this environment lacks make, uv, and pytest.

[seratch]

Closing this PR in favor of #4340 ; your contribution credit is included in its commit. Thanks for reporting the issue and trying to resolve it.