Preserve exception notes when encoding log values

[SAY-5]

Exceptions passed as dictionary values currently lose their PEP 678 notes, even though the exc_info path preserves them. This appends an exception’s own notes to the existing class/message string in order, preserving empty and multiline notes and leaving note-free output unchanged.

Exception groups keep their existing nonrecursive summary and gain their own notes. Child exceptions and their notes are still rendered through exc_info; this change does not add recursive group formatting. Related to #67.

The regression covers dictionary, nested dictionary, and list values through every available formatter, string subclasses, empty/custom exception messages, and group-own notes. On Python 3.13.0, the new tests fail against the unchanged implementation (12 failures, 6 passing controls) and pass with the fix (18 passed); the full available suite passes (171 tests, stdlib JSON and orjson). Black passes. Pylint and mypy were run and report only the same missing msgspec dependency diagnostics as the unchanged baseline. The msgspec backend and other Python/platform versions have not been tested locally.

This PR was prepared with AI assistance (OpenAI Codex).

[nhairs]

Thanks for this @SAY-5.

Before we go much further I want to investigate changes to the __str__ / __repr__ of exceptions as it looks like it's changed in the last few python versions.

Meaning that some of these changes might be redundant, and some of the existing code might also be redundant.