fix: Coerce tool call arguments to match schema types

[janhilgard]

Summary

• Add _coerce_tool_arguments() that auto-stringifies object/array values when the tool schema expects a string
• Fixes a common LLM failure mode where models output raw JSON objects instead of JSON strings (e.g. when writing package.json, the model puts content as an object instead of a string)
• Applied to all tool call emission paths (non-streaming, streaming, streaming fallback)

Problem

When using tool calling with coding assistants (e.g. OpenCoder), models sometimes generate:

{"file_path": "package.json", "content": {"name": "app", "version": "1.0.0"}}

instead of:

{"file_path": "package.json", "content": "{\"name\": \"app\", \"version\": \"1.0.0\"}"}

The client validates against the tool schema (content: string) and fails with:
expected string, received object

Fix

Server-side coercion: after parsing tool call arguments, check each field against the tool schema. If schema says type: "string" but the value is an object/array, json.dumps() it automatically.

Test plan

• Verify tool calls with string content pass through unchanged
• Verify tool calls with object content are auto-stringified
• Verify tool calls with array content are auto-stringified
• Verify no-op when tools schema is not provided
• Verify streaming and non-streaming paths both coerce

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