Fix double rendering of exception tracebacks in stdlib integration

[M97Chahboun]

This PR fixes an issue where exception tracebacks are rendered twice when using structlog's standard library integration with logger.exception().

Previously, if a processor (like format_exc_info or a custom renderer) formatted the exception and added it to the event_dict, the underlying standard library logging.Logger.exception() would still receive exc_info=True by default and render the traceback a second time.

To resolve this, this PR introduces an in-band signaling mechanism. The _proxy_to_logger method in BoundLogger now checks for a _structlog_exception_already_rendered flag in the event dictionary (or within the extra kwargs if render_to_log_kwargs is used). If this flag is present and True, the wrapper suppresses the automatic exception info by explicitly passing exc_info=False to the underlying logger.

This approach perfectly aligns with the discussion in #590, allowing processors to take full control of exception rendering without breaking backward compatibility for standard use cases.

Pull Request Checklist

• I acknowledge this project's AI policy.
• This pull request is not from my main branch. (Note: Please ensure you create a new branch like fix/double-exception-rendering before submitting!)
• There's tests for all new and changed code. (Added tests/test_exc_info_double_render.py)
• New APIs are added to our typing tests in api.py. (N/A: This is an internal behavioral fix, no new public API is exposed.)
• Updated documentation for changed code. (Added a note in the stdlib documentation regarding this behavior.)
• Documentation in .rst and .md files is written using semantic newlines.
• Changes are documented in the changelog. (See snippet below)