fix: add configurable S3 source error handling

[neha-5456]

Summary

Adds configurable error handling to S3Source so document parsing failures can either be raised or skipped without stopping the entire ingestion pipeline.

Fixes #323

Changes

• Added on_error option to S3Source:
  • "raise" — default behavior; parsing errors are propagated.
  • "skip" — logs the parsing failure and continues processing remaining S3 objects.
• Added validation for unsupported on_error values.
• Added warning logging when an object is skipped.
• Ensured S3 access/download errors such as get_object failures are still propagated and are not silently skipped.
• Updated Markdown and CSV decoding so parsing failures follow the configured error-handling policy.
• Added tests covering:
  • Default raise behavior.
  • Skip behavior and continued ingestion.
  • Invalid on_error values.
  • Warning logging.
  • S3 get_object errors.
  • Invalid Markdown front matter.

Testing

uv run --no-sync pytest tests/test_s3_source.py -q

Result: 11 passed

uv run --no-sync ruff check src/dynavec/ingest.py tests/test_s3_source.py

Result: All checks passed!

uv run --no-sync mypy src/dynavec/ingest.py

Result: Success: no issues found in 1 source file