feat(llm): OpenAI provider native function calling

[bug-ops]

Problem

OpenAI provider sends plain text. OpenAI API supports function calling with tool_calls.

Solution

• Serialize ToolDef → OpenAI function format
• Parse tool_calls from response choices
• Return ChatResponse::ToolUse
• supports_tool_use() returns true

Acceptance Criteria

• Function definitions serialized in OpenAI API requests
• tool_calls parsed from responses
• Tool results sent back as function role messages
• Fallback to text mode when no tools provided

Part of #253 (M20)