fix(gemini): resolve 400 errors by strictly adhering to JSON schema

- Remove  from  as it is rejected by the API inside .
- Ensure  is always a JSON object (google.protobuf.Struct), wrapping non-object tool results in .
- Update extraction logic to only look for  in  sibling fields.

src/providers/gemini.rs

@@ -74,8 +74,6 @@ struct GeminiInlineData {
 struct GeminiFunctionCall {
     name: String,
     args: Value,
-    #[serde(skip_serializing_if = "Option::is_none", rename = "thought_signature")]
-    thought_signature: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -264,8 +262,15 @@ impl GeminiProvider {
                     .unwrap_or_default();
 
                 let name = msg.name.clone().or_else(|| msg.tool_call_id.clone()).unwrap_or_else(|| "unknown_function".to_string());
-                let response_value = serde_json::from_str::<Value>(&text_content)
+                
+                // Gemini API requires 'response' to be a JSON object (google.protobuf.Struct).
+                // If it is an array or primitive, wrap it in an object.
+                let mut response_value = serde_json::from_str::<Value>(&text_content)
                     .unwrap_or_else(|_| serde_json::json!({ "result": text_content }));
+                
+                if !response_value.is_object() {
+                    response_value = serde_json::json!({ "result": response_value });
+                }
 
                 parts.push(GeminiPart {
                     text: None,
@@ -328,7 +333,6 @@ impl GeminiProvider {
                             function_call: Some(GeminiFunctionCall {
                                 name: tc.function.name.clone(),
                                 args,
-                                thought_signature: thought_signature.clone(),
                             }),
                             function_response: None,
                             thought: None,
@@ -537,10 +541,9 @@ impl GeminiProvider {
             .filter(|p| p.function_call.is_some())
             .map(|p| {
                 let fc = p.function_call.as_ref().unwrap();
-                // CAPTURE: Try extracting thought_signature from multiple possible locations
+                // CAPTURE: Try extracting thought_signature from sibling fields
                 let id = p.thought_signature_camel.clone()
                     .or_else(|| p.thought_signature_snake.clone())
-                    .or_else(|| fc.thought_signature.clone())
                     .unwrap_or_else(|| format!("call_{}", Uuid::new_v4().simple()));
                 
                 ToolCall {
@@ -886,10 +889,9 @@ impl super::Provider for GeminiProvider {
                                     if let Some(fc) = &p.function_call {
                                         let tool_call_idx = p_idx as u32;
                                         
-                                        // Attempt to find a signature in any possible field
+                                        // Attempt to find a signature in sibling fields
                                         let signature = p.thought_signature_camel.clone()
-                                            .or_else(|| p.thought_signature_snake.clone())
+                                            .or_else(|| p.thought_signature_snake.clone());
-                                            .or_else(|| fc.thought_signature.clone());
 
                                         // Ensure the ID remains stable for this tool call index.
                                         // If we found a real signature now, we update it; otherwise use the existing or new random ID.