refactor(llm): unify ToolError contract for arg validation

[rosetta-livekit-bot]

Summary

Unifies tool-argument validation under a single ToolError contract owned by prepare_function_arguments, and fixes a malformed-JSON propagation bug along the way.

• prepare_function_arguments wraps ValidationError / ValueError / TypeError as ToolError("Error parsing arguments for ..."), so the message the LLM sees is owned in one place.
• Accepts an optional fnc_call: FunctionCall | None parameter. When provided and json_arguments is a string, the canonical JSON (post json_repair) is written back to fnc_call.arguments before pydantic validation runs. This way, even when validation later fails, the conversation history holds valid JSON instead of the broken raw payload — without this, providers like Vertex/OpenAI reject the next request with a 5xx when re-serializing it (#5807 review).
• execute_function_call drops its dedicated (ValidationError, ValueError) branch — one prepare_function_arguments(..., fnc_call=fnc_call) call covers both parse failures and validation failures.
• ToolProxyToolset._handle_call drops its own try/except wrapper and passes the parameters dict directly (no extra json.dumps).
• Pure-validation callers (judge.py, async_toolset.py, run_result.py) are unchanged — they just don't pass fnc_call.

Extracted from #5711.

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Latest commit: 2637fbd

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[devin-ai-integration]

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