feat: add tool call support to chat completions endpoint

- Accept tools and tool_choice fields in CompletionRequest
- Convert OpenAI tools to Gemini format and store in MitmStore
- Detect MITM-captured function calls in streaming poll loop
- Emit tool_calls delta chunks in OpenAI streaming format
- Finish with 'tool_calls' reason instead of 'stop' when tools used
- Only clear tools when request has none (prevents stale state leak)

src/api/types.rs

@@ -49,6 +49,10 @@ pub(crate) struct CompletionRequest {
     pub stream: bool,
     #[serde(default = "default_timeout")]
     pub timeout: u64,
+    /// OpenAI-format tool definitions
+    pub tools: Option<Vec<serde_json::Value>>,
+    /// Tool choice: "auto", "none", "required", or {"type":"function","function":{"name":"..."}}
+    pub tool_choice: Option<serde_json::Value>,
 }
 
 #[derive(Deserialize)]