Fixes #172

Summary

This updates firebase_functions.logger so ERROR logs can safely include exception values and other non-JSON-safe payloads without failing serialization, while aligning the emitted traceback shape with the JS SDK. Stack traces now surface at top-level stack_trace, and the Python-only structured exception surface introduced earlier in the PR is removed.

Problem/Root Cause

The original logger could fail when error metadata included exception-related values that were not directly JSON serializable, including the documented sys.exc_info()[0] pattern and exception arguments containing opaque objects, bad repr() implementations, or cyclic data.

The initial fix in this PR added a Python-specific structured exception payload under error and a new logger.exception(...) helper. Reviewer feedback called out that this diverged from the JS SDK's logging shape, especially around traceback placement and the public API surface.

Solution/Changes

The logger now applies the defensive serialization work generically, keeping circular-reference handling and safe repr coercion for arbitrary metadata values instead of treating exceptions as a special nested schema.

For ERROR severity, the logger inspects the provided arguments and active exception state, then emits any available traceback at top-level stack_trace. Exception values passed via error= remain ordinary serialized metadata, so opaque or non-JSON-safe exception arguments no longer crash logging, but logs no longer emit nested-only traces or a Python-only logger.exception(...) helper.

The test coverage was updated to match the final behavior, including exception instances, sys.exc_info()[0], self-referential and cyclic payloads, non-JSON-safe values, and the top-level stack trace shape.

Testing

• Updated tests/test_logger.py to cover exception instances, exception types from sys.exc_info()[0], cyclic/self-referential payloads, non-JSON-safe values, and top-level stack_trace behavior for ERROR logs.
• Ran python3 -m pytest -q tests/test_logger.py