feat: add tool-calling passthrough for all providers

Implement full OpenAI-compatible tool-calling support across the proxy,
enabling OpenCode to use llm-proxy as its sole LLM backend.

- Add 9 tool-calling types (Tool, FunctionDef, ToolChoice, ToolCall, etc.)
- Update ChatCompletionRequest/ChatMessage/ChatStreamDelta with tool fields
- Update UnifiedRequest/UnifiedMessage to carry tool data through the pipeline
- Shared helpers: messages_to_openai_json handles tool messages, build_openai_body
  includes tools/tool_choice, parse/stream extract tool_calls from responses
- Gemini: full OpenAI<->Gemini format translation (functionDeclarations,
  functionCall/functionResponse, synthetic call IDs, tool_config mapping)
- Gemini: extract duplicated message-conversion into shared convert_messages()
- Server: SSE streams include tool_calls deltas, finish_reason='tool_calls'
- AggregatingStream: accumulate tool call deltas across stream chunks
- OpenAI provider: add o4- prefix to supports_model()

src/providers/openai.rs

@@ -36,7 +36,7 @@ impl super::Provider for OpenAIProvider {
     }
 
     fn supports_model(&self, model: &str) -> bool {
-        model.starts_with("gpt-") || model.starts_with("o1-") || model.starts_with("o3-")
+        model.starts_with("gpt-") || model.starts_with("o1-") || model.starts_with("o3-") || model.starts_with("o4-")
     }
 
     fn supports_multimodal(&self) -> bool {