feat: add support for Gemini thought_signature in tool calls

Gemini 3.x models require a thought_signature to be returned in subsequent
conversation turns when using tool calls. This patch adds the necessary
fields to ToolCall protocol types and updates the OpenAI-compatible provider
and agent loop to capture and preserve these signatures.

pkg/agent/loop.go

@@ -604,6 +604,7 @@ func (al *AgentLoop) runLLMIteration(ctx context.Context, agent *AgentInstance,
 					Arguments: string(argumentsJSON),
 				},
 				Name:         tc.Name,
+				ExtraContent: tc.ExtraContent,
 			})
 		}
 		messages = append(messages, assistantMsg)

pkg/providers/openai_compat/provider.go

@@ -133,6 +133,11 @@ func parseResponse(body []byte) (*LLMResponse, error) {
 						Name      string `json:"name"`
 						Arguments string `json:"arguments"`
 					} `json:"function"`
+					ExtraContent *struct {
+						Google *struct {
+							ThoughtSignature string `json:"thought_signature"`
+						} `json:"google"`
+					} `json:"extra_content"`
 				} `json:"tool_calls"`
 			} `json:"message"`
 			FinishReason string `json:"finish_reason"`
@@ -167,10 +172,20 @@ func parseResponse(body []byte) (*LLMResponse, error) {
 			}
 		}
 
+		var extra *protocoltypes.ToolCallExtraContent
+		if tc.ExtraContent != nil && tc.ExtraContent.Google != nil {
+			extra = &protocoltypes.ToolCallExtraContent{
+				Google: &protocoltypes.GoogleExtraContent{
+					ThoughtSignature: tc.ExtraContent.Google.ThoughtSignature,
+				},
+			}
+		}
+
 		toolCalls = append(toolCalls, ToolCall{
 			ID:           tc.ID,
 			Name:         name,
 			Arguments:    arguments,
+			ExtraContent: extra,
 		})
 	}
 

pkg/providers/protocoltypes/types.go

@@ -6,6 +6,15 @@ type ToolCall struct {
 	Function     *FunctionCall          `json:"function,omitempty"`
 	Name         string                 `json:"name,omitempty"`
 	Arguments    map[string]interface{} `json:"arguments,omitempty"`
+	ExtraContent *ToolCallExtraContent  `json:"extra_content,omitempty"`
+}
+
+type ToolCallExtraContent struct {
+	Google *GoogleExtraContent `json:"google,omitempty"`
+}
+
+type GoogleExtraContent struct {
+	ThoughtSignature string `json:"thought_signature,omitempty"`
 }
 
 type FunctionCall struct {