fix(gemini): align thought_signature logic with Gemini API requirements

- Extract thought_signature from the Part level during response parsing.
- Provide thought_signature as a sibling to functionCall during request assembly.
- This fully resolves the 'Unknown name thoughtSignature at function_call' error.

src/providers/gemini.rs

@@ -57,6 +57,8 @@ struct GeminiPart {
     function_response: Option<GeminiFunctionResponse>,
     #[serde(skip_serializing_if = "Option::is_none")]
     thought: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    thought_signature: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -70,8 +72,6 @@ struct GeminiInlineData {
 struct GeminiFunctionCall {
     name: String,
     args: Value,
-    #[serde(skip_serializing_if = "Option::is_none")]
-    thought_signature: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -231,6 +231,7 @@ impl GeminiProvider {
                                 function_call: None,
                                 function_response: None,
                                 thought: None,
+                                thought_signature: None,
                             });
                         }
                     }
@@ -270,6 +271,7 @@ impl GeminiProvider {
                         response: response_value,
                     }),
                     thought: None,
+                    thought_signature: None,
                 });
             } else if msg.role == "assistant" {
                 // Assistant messages: handle text, thought (reasoning), and tool_calls
@@ -282,6 +284,7 @@ impl GeminiProvider {
                                 function_call: None,
                                 function_response: None,
                                 thought: None,
+                                thought_signature: None,
                             });
                         }
                     }
@@ -296,6 +299,7 @@ impl GeminiProvider {
                             function_call: None,
                             function_response: None,
                             thought: Some(reasoning.clone()),
+                            thought_signature: None,
                         });
                     }
                 }
@@ -318,10 +322,10 @@ impl GeminiProvider {
                             function_call: Some(GeminiFunctionCall {
                                 name: tc.function.name.clone(),
                                 args,
-                                thought_signature,
                             }),
                             function_response: None,
                             thought: None,
+                            thought_signature,
                         });
                     }
                 }
@@ -337,6 +341,7 @@ impl GeminiProvider {
                                     function_call: None,
                                     function_response: None,
                                     thought: None,
+                                    thought_signature: None,
                                 });
                             }
                         }
@@ -355,6 +360,7 @@ impl GeminiProvider {
                                 function_call: None,
                                 function_response: None,
                                 thought: None,
+                                thought_signature: None,
                             });
                         }
                     }
@@ -390,6 +396,7 @@ impl GeminiProvider {
                         function_call: None,
                         function_response: None,
                         thought: None,
+                        thought_signature: None,
                     }],
                 });
             }
@@ -516,10 +523,11 @@ impl GeminiProvider {
     fn extract_tool_calls(parts: &[GeminiPart]) -> Option<Vec<ToolCall>> {
         let calls: Vec<ToolCall> = parts
             .iter()
-            .filter_map(|p| p.function_call.as_ref())
+            .filter(|p| p.function_call.is_some())
-            .map(|fc| {
+            .map(|p| {
-                // CAPTURE: Use thought_signature as the ID if available
+                let fc = p.function_call.as_ref().unwrap();
-                let id = fc.thought_signature.clone().unwrap_or_else(|| format!("call_{}", Uuid::new_v4().simple()));
+                // CAPTURE: Use thought_signature from Part as the ID if available
+                let id = p.thought_signature.clone().unwrap_or_else(|| format!("call_{}", Uuid::new_v4().simple()));
                 
                 ToolCall {
                     id,
@@ -539,11 +547,12 @@ impl GeminiProvider {
     fn extract_tool_call_deltas(parts: &[GeminiPart]) -> Option<Vec<ToolCallDelta>> {
         let deltas: Vec<ToolCallDelta> = parts
             .iter()
-            .filter_map(|p| p.function_call.as_ref())
             .enumerate()
-            .map(|(i, fc)| {
+            .filter(|(_, p)| p.function_call.is_some())
-                // CAPTURE: Use thought_signature as the ID if available
+            .map(|(i, p)| {
-                let id = fc.thought_signature.clone().unwrap_or_else(|| format!("call_{}", Uuid::new_v4().simple()));
+                let fc = p.function_call.as_ref().unwrap();
+                // CAPTURE: Use thought_signature from Part as the ID if available
+                let id = p.thought_signature.clone().unwrap_or_else(|| format!("call_{}", Uuid::new_v4().simple()));
 
                 ToolCallDelta {
                     index: i as u32,