test(proxy): add adversarial DAP scenario fixture

[debugmcpdev]

Summary

• add a deterministic TCP DAP scenario fixture behind the existing internal ProxyConfig.adapterCommand seam
• cover dropped and delayed responses, event ordering, junk-stream resynchronization, duplicate responses, controlled connection closure, and adapters that never listen
• make the proxy initialization deadline injectable for resilience tests while retaining the 30-second production default
• document the fixture and its JSON scenario controls

Closes #511

Validation

• pnpm run build
• pnpm run lint
• pnpm run test:unit (4,272 passed)
• pnpm run test:integration (28 passed, 5 skipped)
• pnpm run check:personal-paths
• pnpm run check:all-personal-paths
• git diff --check
• node --check tests/fixtures/adversarial-adapter/server.mjs

The new integration suite drives the compiled proxy bootstrap, worker IPC, adapter process manager, real loopback TCP socket, MinimalDapClient, and shared DAP framing decoder. It verifies:

• recovery when initialized arrives but the initialize response is absent
• stage-aware diagnostics when launch is the stalled request
• recovery from junk immediately followed by a valid response, while tolerating delayed events and a duplicate response
• prompt failure on a mid-frame socket close
• distinct diagnostics when the adapter spawned but never established its listening socket

Dogfooding

Used the source dev proxy to debug the new fixture itself with the JavaScript adapter. A statement breakpoint in the scenario dispatcher exposed the raw request and selected rule; locals and expression evaluation confirmed the delay, junk-prefix, and duplicate-response controls before stepping and continuing. A raw TCP request then received the fixture's adversarial output. The only shutdown warning reproduced the already-known #527 behavior covered by #542; no new unique defect was found.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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