fix(gemini): preserve thought_signature in tool calls to prevent 400 errors

pkg/agent/loop.go

@@ -624,12 +624,18 @@ func (al *AgentLoop) runLLMIteration(ctx context.Context, messages []providers.M
 		}
 		for _, tc := range response.ToolCalls {
 			argumentsJSON, _ := json.Marshal(tc.Arguments)
+			thoughtSignature := ""
+			if tc.Function != nil {
+				thoughtSignature = tc.Function.ThoughtSignature
+			}
+
 			assistantMsg.ToolCalls = append(assistantMsg.ToolCalls, providers.ToolCall{
 				ID:   tc.ID,
 				Type: "function",
 				Function: &providers.FunctionCall{
-					Name:      tc.Name,
-					Arguments: string(argumentsJSON),
+					Name:             tc.Name,
+					Arguments:        string(argumentsJSON),
+					ThoughtSignature: thoughtSignature,
 				},
 			})
 		}