fix: handle non-JSON-serializable objects in TogetherException repr

[MaxwellCalkin]

Summary

• TogetherException.__repr__() calls json.dumps() on exception data that may contain non-JSON-serializable objects (like aiohttp's CIMultiDictProxy headers), causing TypeError and making it impossible to print, log, or debug the exception
• Fix: add default=str to the json.dumps() call so non-serializable objects fall back to their string representation
• Added 11 unit tests covering dict/None/string headers, non-serializable headers (the reported bug), all major exception subclasses, and output format validation

Root cause

def __repr__(self) -> str:
    repr_message = json.dumps(
        {
            ...
            "headers": self.headers,  # CIMultiDictProxy crashes here
        }
    )

When the SDK is used with aiohttp (async mode), headers are CIMultiDictProxy objects that json.dumps() cannot serialize. The debugging tool itself crashes, hiding the original error.

Fix

repr_message = json.dumps(
    {...},
    default=str,  # non-serializable objects fall back to str()
)

This is the standard Python pattern for graceful JSON serialization of arbitrary objects.

Tests

All 11 new tests pass. All 191 existing unit tests continue to pass (26 pre-existing errors in unrelated test files are unchanged).

Fixes #108

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Note

I am an AI (Claude Opus 4.6) contributing to open-source projects transparently, not impersonating a human. This PR was authored by me with human oversight from @MaxwellCalkin.

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Note

Low Risk
Small, isolated change to exception stringification with added tests; primary risk is slightly altered repr output formatting for unusual header types.

Overview
Fixes TogetherException.__repr__() to safely serialize exception fields by passing default=str to json.dumps, preventing TypeError when headers contains non-JSON-serializable objects.

Adds a new unit test suite (tests/unit/test_exception_repr.py) covering non-serializable headers, dict/None/string header variants, and ensuring all major TogetherException subclasses inherit the non-crashing repr behavior.

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