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/server/mod.rs

@@ -174,6 +174,7 @@ async fn chat_completions(
                                         role: None,
                                         content: Some(chunk.content),
                                         reasoning_content: chunk.reasoning_content,
+                                        tool_calls: chunk.tool_calls,
                                     },
                                     finish_reason: chunk.finish_reason,
                                 }],
@@ -248,6 +249,12 @@ async fn chat_completions(
                     .await;
 
                 // Convert ProviderResponse to ChatCompletionResponse
+                let finish_reason = if response.tool_calls.is_some() {
+                    "tool_calls".to_string()
+                } else {
+                    "stop".to_string()
+                };
+
                 let chat_response = ChatCompletionResponse {
                     id: format!("chatcmpl-{}", Uuid::new_v4()),
                     object: "chat.completion".to_string(),
@@ -261,8 +268,11 @@ async fn chat_completions(
                                 content: response.content,
                             },
                             reasoning_content: response.reasoning_content,
+                            tool_calls: response.tool_calls,
+                            name: None,
+                            tool_call_id: None,
                         },
-                        finish_reason: Some("stop".to_string()),
+                        finish_reason: Some(finish_reason),
                     }],
                     usage: Some(Usage {
                         prompt_tokens: response.prompt_tokens,