fix(streaming): add helpful error message for malformed JSON in non-beta path

[Vitalcheffe]

What

Wraps the from_json() call in the non-beta streaming path with a try-except, providing the same helpful error message that already exists in the beta path.

Why

When using client.messages.stream() with tools, if the model emits malformed JSON during an input_json_delta event, the user gets a raw ValueError: expected ident at line 1 column 11 with zero context. The beta path (_beta_messages.py) already handles this gracefully with a clear message. The non-beta path (_messages.py) was simply missing the same error handling.

See: #1265

How

1. _messages.py: Wrapped the from_json(json_buf, partial_mode=True) call in a try: ... except ValueError as e: block. On error, raises a new ValueError with a descriptive message including the original error and the JSON that failed to parse, chained via from e.
2. test_non_beta_partial_json.py: Added three regression tests:
   • test_valid_json_parses_correctly — ensures valid JSON still works
   • test_invalid_json_raises_helpful_error — verifies the error message is user-friendly
   • test_invalid_json_chained_from_original — verifies error chaining for debugging

Testing

• ruff check passes on all files
• Tests match existing patterns from test_partial_json.py (beta path)

Checklist

• Tests added
• Linter passes
• No breaking change (same error type, just better message)

Closes #1265

[Vitalcheffe]

I’m aware _messages.py may be auto-generated via Stainless. If that’s the case, I’m happy to discuss where the fix should live upstream, or whether there’s a manual override mechanism for exception handling in the generator config. Open to guidance on the preferred approach.