fix(mcp): preserve live graph request cancellation

[ScriptedAlchemy]

Summary

Stop a cancelled MCP graph request's blocking worker from reading another file after a subsequent ping has acknowledged cancellation.

Motivation

The caller's MCP cancellation signal was relayed asynchronously into a native request token and then into a reconstructed graph signal. A ping could finish before those relays ran, allowing a paused rename-preview worker to resume with an uncancelled replacement signal.

Changes

• Retain the caller's live signal in the existing daemon request cancellation registry and retrieve it only for the matching native token.
• Keep the request lease through in-process settlement, preserving daemon cancellation, deadlines, request isolation, and the existing typed invalid-request result for duplicate IDs.
• Cover immediate signal visibility without polling a relay, daemon cancellation, isolation, and cancellation before registration. The existing strict rename-preview tests and all budgets remain unchanged.

No wire or persisted contracts change. No LSP source changes are included.

Verification

All checks ran through Bazel on this commit:

• Strict transport-cancellation regression: 20/20 runs passed, each executing exactly one test with the original one-source-read assertion.
• Strict deadline regression: 1 passed, with the existing anti-vacuity helper.
• Daemon request-cancellation registry tests: 10 passed.
• Affected daemon-service and composition-root rustfmt checks: passed.
• Clippy for daemon-service unit tests and the MCP suite, with the repository's warnings-as-errors configuration: passed.
• Native LSP refresh regression: 10/10 exact runs passed; full application unit suite: 510 passed.
• One additional native LSP protocol trace passed: initialize write to Python response 110.270 ms, response to native stdout read 0.151 ms, child termination to reap 1.720 ms. Stdout reached EOF before test exit. The roughly five-second successful duration is the production diagnostics quiet window, distinct from the initialization budget. The historical initialize timeout did not reproduce and its cause remains unproven.

Commands used --remote_download_outputs=all for these invocations after the initial build encountered missing remote CAS artifacts; no repository cache configuration changed. A full workspace build is not claimed. Focused checks cover the changed daemon-service/composition-root boundary and the reported native LSP behavior.

Checklist

• Original strict assertions and timeouts preserved
• Relevant behavioral tests, formatting, and Clippy passed
• No secrets, credentials, or environment files included
• No wire or persisted contract changes

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[devin-ai-integration]

Devin Review found 1 potential issue.