feat: add structured output to Gemini endpoint

Gemini endpoint now accepts responseMimeType and responseSchema
fields, injected into Google's generationConfig via MITM. Supports
both snake_case and camelCase aliases.

src/api/gemini.rs

@@ -67,6 +67,14 @@ pub(crate) struct GeminiRequest {
     /// When true, injects {"googleSearch": {}} into tools via MITM.
     #[serde(default, alias = "googleSearch")]
     pub google_search: bool,
+    /// Response MIME type for structured output, e.g. "application/json".
+    /// Injected as generationConfig.responseMimeType via MITM.
+    #[serde(default, alias = "responseMimeType")]
+    pub response_mime_type: Option<String>,
+    /// Response schema for structured output (JSON Schema object).
+    /// Injected as generationConfig.responseSchema via MITM.
+    #[serde(default, alias = "responseSchema")]
+    pub response_schema: Option<serde_json::Value>,
 }
 
 fn default_timeout() -> u64 {
@@ -257,8 +265,8 @@ pub(crate) async fn handle_gemini(
             frequency_penalty: None,
             presence_penalty: None,
             reasoning_effort: body.thinking_level.clone(),
-            response_mime_type: None,
+            response_mime_type: body.response_mime_type.clone(),
-            response_schema: None,
+            response_schema: body.response_schema.clone(),
             google_search: body.google_search,
         };
         if gp.temperature.is_some()
@@ -267,6 +275,8 @@ pub(crate) async fn handle_gemini(
             || gp.max_output_tokens.is_some()
             || gp.stop_sequences.is_some()
             || gp.reasoning_effort.is_some()
+            || gp.response_mime_type.is_some()
+            || gp.response_schema.is_some()
             || gp.google_search
         {
             state.mitm_store.set_generation_params(gp).await;