fix buffered inline comment delivery failures

[lakshayxi]

Fixes #1679.

Buffered inline comment delivery failures were logged but otherwise ignored, so a review could report Posted 0/N and still succeed.

This change makes failed posts visible with GitHub Actions error annotations and preserves dropped comment bodies in the logs. Partial delivery remains non-fatal so successfully posted findings are still usable, while a complete 0/N delivery failure exits non-zero after all comments have been attempted.

Tests cover successful delivery, partial and total failures, GitHub 422 diagnostics, multiline comment logging, and unserializable API error data.

Validation:

• bun test test/post-buffered-inline-comments.test.ts
• bun test
• bun run typecheck
• bun run format:check
• git diff --check