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/dashboard/providers.rs

@@ -325,11 +325,16 @@ pub(super) async fn handle_test_provider(
         messages: vec![crate::models::UnifiedMessage {
             role: "user".to_string(),
             content: vec![crate::models::ContentPart::Text { text: "Hi".to_string() }],
+            tool_calls: None,
+            name: None,
+            tool_call_id: None,
         }],
         temperature: None,
         max_tokens: Some(5),
         stream: false,
         has_images: false,
+        tools: None,
+        tool_choice: None,
     };
 
     match provider.chat_completion(test_request).await {