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/mod.rs

@@ -50,6 +50,7 @@ pub trait Provider: Send + Sync {
 pub struct ProviderResponse {
     pub content: String,
     pub reasoning_content: Option<String>,
+    pub tool_calls: Option<Vec<crate::models::ToolCall>>,
     pub prompt_tokens: u32,
     pub completion_tokens: u32,
     pub total_tokens: u32,
@@ -61,6 +62,7 @@ pub struct ProviderStreamChunk {
     pub content: String,
     pub reasoning_content: Option<String>,
     pub finish_reason: Option<String>,
+    pub tool_calls: Option<Vec<crate::models::ToolCallDelta>>,
     pub model: String,
 }