* refactor: remove _SentryFriendlyProcessorFormatter, use structlog's built-in record copy

structlog >=24.1 makes a shallow copy of the LogRecord in
ProcessorFormatter.format(), so the original msg, args, and exc_info
are preserved for Sentry's LoggingIntegration hook. No need to
snapshot/restore them ourselves.

- Inline formatter construction into setup_structlog()
- Use pass_foreign_args=True for gunicorn access processor
- Add format_exc_info to foreign_pre_chain for stdlib logs
- Strip positional_args in map_event_to_json_record
- Assert exact tracebacks and Sentry events in tests

* refactor: remove unused JsonFormatter class

No production consumer — all JSON formatting goes through
map_event_to_json_record via structlog's ProcessorFormatter.
JsonRecord TypedDict stays as the schema definition.

* docs: explain why positional_args is stripped in map_event_to_json_record

* docs: remove unnecessary Sentry note from setup_structlog docstring

* docs: explain why positional_args is needed in gunicorn access processor

updates:
- [github.com/astral-sh/ruff-pre-commit: v0.15.5 → v0.15.7](astral-sh/ruff-pre-commit@v0.15.5...v0.15.7)
- [github.com/astral-sh/uv-pre-commit: 0.10.9 → 0.10.12](astral-sh/uv-pre-commit@0.10.9...0.10.12)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>