test(mcp): prove tracedecay_feedback_expand behavior

[ScriptedAlchemy]

Summary

• Proves tracedecay_feedback_expand through the production MCP server, not a schema or handler mock.
• A published compiler diagnostic is expanded by its daemon-minted handle. The tool returns that finding and its one diagnostic anchor.
• A missing argument, a blank handle, an unknown handle, and a diagnostics handle are refused with the typed errors a caller actually sees.

Motivation

The expand tool is handle-gated. Advertising the request schema does not show that a real handle hydrates the retained diagnostic, or that the other handles fail closed.

Changes

• crates/tracedecay/tests/mcp_suite/mcp_handler_test/feedback_expand_test.rs calls the production tool and asserts literal payloads.

Test plan

• scripts/require-exact-test.sh cargo test -p tracedecay --features test-transport --test mcp_suite mcp_handler_test::feedback_expand_test::feedback_expand_returns_the_published_diagnostic_and_denies_other_handles -- --exact — passed locally: ok. 1 passed; 0 failed; 350 filtered out; finished in 2.15s
• Do not merge. This PR only owns tracedecay_feedback_expand.

Checklist

• No secrets, credentials, or .env files included
• No changelog: test-only, no product behavior change
• No breaking changes

  

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