test: add regression tests for observability and privacy features

[p70436464-prog]

Closes #1556

Description

Addresses the objective of establishing a durable regression contract for normal, boundary, accessibility, and compatibility behavior for the privacy-safe observability and tracing implementation.

Acceptance Criteria Satisfied:

• The implementation defines and enforces the relevant invariants for normal and adversarial inputs.
• Added focused unit and integration tests for success, failure, loading, empty, retry, and permission states.
• Documented the supported API/component contract (via test suites acting as a regression contract) and protected existing consumers from accidental breaking changes.
• Automated tests cover success, failure, boundary, retry, and permission behavior applicable to this feature.
• N/A: Verify keyboard, focus, screen-reader, responsive, and reduced-motion behavior where the feature is interactive (these are backend utility middleware).

Validation Commands

• Run npm test or node --experimental-vm-modules node_modules/jest/bin/jest.js src/middleware/privacy-logger.test.ts src/observability/tracing.test.ts src/observability/httpMetrics.test.ts to execute these specific tests.

Design Tradeoffs & Limitations

• The middleware directly hooks into the res.on('finish') event, which is correct for logging and metrics but requires careful teardown and error handling logic, which we’ve explicitly tested.
• IP masking and Redaction algorithms are built for speed and strict coverage but have known tradeoffs in identifying complex recursive shapes (partially mitigated by test assertions on cycle handling).