Add diagnostic metadata regression tests

[Danservfinn]

Summary

Adds focused diagnostic metadata regression coverage for the redaction/path-normalization contract requested in #1, and tightens the build diagnostic metadata generation so PR JSON does not leak local checkout, home, temp, user, or host details.

Fixes #1.

Changes

• Added deterministic unittest coverage for repository-relative / artifact paths.
• Added metadata redaction for home, temp, repo checkout, user, and host values in diagnostic JSON output.
• Added diagnostic artifact pair validation with clear failures for missing JSON, missing .logd, non-relative references, and mismatches.
• Kept build diagnostics deterministic when encryptly preflight fails by committing the JSON blocker instead of retrying a known-failed pack path.
• Included diagnostic/build-c2c53e9d.json from the required local python3 build.py validation run. The encrypted .logd could not be produced because encryptly preflight timed out in this environment; the JSON records that blocker.

Testing

• python3 -m unittest tests.test_diagnostic_metadata -v
  • Result: 4 tests passed.
• python3 build.py
  • Result: exited 1 because encryptly preflight timed out after 60s in this environment.
  • The script failed clearly, created diagnostic/build-c2c53e9d.json, and committed the diagnostic JSON blocker.

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build log is committed in this PR
• Documentation has been updated, if applicable
• Configuration or schema changes are documented, if applicable
• No generated build artifacts are committed, except the required diagnostic build log
• Changes are scoped to the PR purpose and avoid unrelated cleanup
• Security, privacy, and error-handling implications have been considered

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• I would like to request that my diagnostic build log is removed before merging