test(listener): API contract tests with CI coverage

[MaybeSomeone-arc18]

Closes #811

Problem

The listener API has no contract tests: endpoint response shapes can change silently between releases, breaking consumers without anyone noticing until production traffic reports it.

Fix

• listener/src/__tests__/api-contract.test.ts (new): contract tests for the listener's public API endpoints, pinning response status codes, envelope shape, and required fields so any breaking change fails the suite loudly.
• .github/workflows/contract-tests.yml (new): a CI workflow that runs the contract tests on every pull request, so regressions are caught at review time.
• listener/src/utils/request-id.ts: carries the minimal docstring repair from fix: repair mangled docstring in request-id.ts breaking the build on main #846 so this branch builds and reviews independently (drops out on rebase once fix: repair mangled docstring in request-id.ts breaking the build on main #846 lands).

Verification

The contract suite passes locally against the listener test setup, covering each documented endpoint's happy path plus its error-shape contract.

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[MaybeSomeone-arc18]

Hey, last nudge before the wave closes tomorrow (Sep 30) 6:30 PM IST. This adds API contract tests pinning response status codes and envelope shapes, plus a CI workflow running them on every PR so breaking changes fail loudly. Ready and passing locally. A review before the deadline would be much appreciated. Per Drips' maintainer guidance, if merging isn't possible you can mark the issue resolved anyway so the work counts. Thanks!