feat: full tool call support (OpenAI + Gemini endpoints)

- store.rs: Add tool context storage (active tools, tool config, pending
  tool results, call_id mapping, last function calls for history rewrite)
- types.rs: Add tools/tool_choice fields to ResponsesRequest, add
  build_function_call_output helper for OpenAI function_call output items
- modify.rs: Replace hardcoded get_weather with dynamic ToolContext
  injection. Add openai_tools_to_gemini and openai_tool_choice_to_gemini
  converters. Add conversation history rewriting for tool result turns
  (replaces fake 'Tool call completed' model turn with real functionCall,
  injects functionResponse before last user turn)
- proxy.rs: Build ToolContext from MitmStore before calling modify_request.
  Save last_function_calls for history rewriting on subsequent turns
- responses.rs: Store client tools in MitmStore before LS call. Detect
  function_call_output in input array for tool result submission. Return
  captured functionCalls as OpenAI function_call output items with
  generated call_ids and stringified arguments
- gemini.rs: New Gemini-native endpoint (POST /v1/gemini) with zero
  format translation. Accepts functionDeclarations directly, returns
  functionCall in Gemini format directly
- mod.rs: Wire /v1/gemini route, bump version to 3.3.0

src/api/types.rs

@@ -32,6 +32,12 @@ pub(crate) struct ResponsesRequest {
     pub metadata: Option<serde_json::Value>,
     #[serde(default)]
     pub user: Option<String>,
+    /// Tool definitions (OpenAI format).
+    #[serde(default)]
+    pub tools: Option<Vec<serde_json::Value>>,
+    /// Tool choice: "auto", "required", "none", or {"type":"function","function":{"name":"X"}}.
+    #[serde(default)]
+    pub tool_choice: Option<serde_json::Value>,
 }
 
 /// Chat Completions request (OpenAI-compatible).
@@ -220,6 +226,18 @@ pub fn build_message_output_in_progress(msg_id: &str) -> serde_json::Value {
     })
 }
 
+/// Build a function_call output item (OpenAI Responses API format).
+pub fn build_function_call_output(call_id: &str, name: &str, arguments: &str) -> serde_json::Value {
+    serde_json::json!({
+        "type": "function_call",
+        "id": call_id,
+        "call_id": call_id,
+        "name": name,
+        "arguments": arguments,
+        "status": "completed",
+    })
+}
+
 // ─── Helpers ─────────────────────────────────────────────────────────────────
 
 /// Serialize Option<u64> as either the number or JSON null (not omitted).