Fix #277: MCP tool calls honor abort_controller (ESC-cancel)

[ericleepi314]

Closes #277
Closes #171 (duplicate)

Stacked on #307 (← #306 ← #305 ← #304). Merge in order; GitHub retargets automatically.

Summary

ESC during a long-running MCP tool hung until the MCP request timeout (5 min default). The pending JSON-RPC future now has a real cancellation path:

• McpClient._send_request / call_tool accept an abort_signal. The abort listener fires on the aborting thread (TUI ESC handler) and hops onto the request loop via call_soon_threadsafe(future.cancel) — the call unblocks in ~0.1s.
• MCP notifications/cancelled is sent best-effort with the in-flight requestId so a compliant server stops the work — bounded to 2s so a fully wedged transport (the likely cause of the hang being escaped) can't block the unblock path.
• Cleanup is exhaustive: a finally pops the pending entry and removes the listener across all exit paths — success, timeout, abort, genuine task-cancel, and a transport.send that raises or is cancelled mid-await (the send was moved inside the try after review caught the leak).
• Receive-loop hardening: a response racing the abort cleanup lands on an already-cancelled future; resolving it would raise InvalidStateError and kill the loop (nuking every pending request). It's now skipped via a done() check.
• Tool wrapper: re-raises AbortError past its except Exception so the dispatch layer renders the user-cancel message instead of a generic tool error; session-expiry retry threads the signal too.

Scope note: list_tools/list_resources/initialize are startup-time, never on the interactive ESC path — deliberately not signal-threaded.

Test plan

• 9 new tests in tests/test_mcp_abort.py: abort unblocks <2s on a hanging transport; cancellation notification carries the right requestId (and id=None); pending cleaned; pre-aborted sends nothing; listener removed on normal completion; send-failure cleans listener+pending; receive loop survives a late response for a cancelled future; wrapper re-raises AbortError
• All 531 MCP-related tests pass (5 session-expiry stubs updated for the new kwarg)
• Full suite on the stack: 7794 passed, 0 failed, 5 skipped
• Critic review loop: APPROVE after 1 revision round (send-inside-try listener leak + bounded notification both came out of it, empirically verified by the reviewer)

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