Fix unhandled JSONDecodeError in AnthropicClient tool call parsing

[hobostay]

Summary

• json.loads(args) in _convert_messages was called without a try/except when parsing tool call arguments
• Malformed JSON in arguments would raise an unhandled json.JSONDecodeError, crashing the inference loop with an unhelpful traceback
• Wrap in try/except and fall back to empty dict on parse failure, consistent with LlamafileClient's handling

Impact

• Without this fix, malformed tool call arguments from upstream systems would crash the entire inference loop instead of producing a recoverable error

Test plan

• Verify malformed JSON in tool call arguments doesn't crash the client
• Verify valid JSON arguments still parse correctly
• Run existing test suite

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