fix(gemini): 导出 DummyThoughtSignature 常量并修复跨账号签名验证

- 将 dummyThoughtSignature 改为导出的 DummyThoughtSignature 常量，供跨包使用
- 修改 gemini_native_signature_cleaner.go 将删除签名改为替换为 dummy 签名
  这样可以跳过 Gemini 3 的签名验证，解决粘性会话切换账号时的验证失败问题
- 更新相关测试文件

Fixes: 粘性会话切换账号时 thoughtSignature 验证失败导致 400 错误

backend/internal/pkg/antigravity/request_transformer.go

@@ -312,7 +312,7 @@ func buildContents(messages []ClaudeMessage, toolIDToName map[string]string, isT
 				parts = append([]GeminiPart{{
 					Text:             "Thinking...",
 					Thought:          true,
-					ThoughtSignature: dummyThoughtSignature,
+					ThoughtSignature: DummyThoughtSignature,
 				}}, parts...)
 			}
 		}
@@ -330,9 +330,10 @@ func buildContents(messages []ClaudeMessage, toolIDToName map[string]string, isT
 	return contents, strippedThinking, nil
 }
 
-// dummyThoughtSignature 用于跳过 Gemini 3 thought_signature 验证
+// DummyThoughtSignature 用于跳过 Gemini 3 thought_signature 验证
 // 参考: https://ai.google.dev/gemini-api/docs/thought-signatures
-const dummyThoughtSignature = "skip_thought_signature_validator"
+// 导出供跨包使用（如 gemini_native_signature_cleaner 跨账号修复）
+const DummyThoughtSignature = "skip_thought_signature_validator"
 
 // buildParts 构建消息的 parts
 // allowDummyThought: 只有 Gemini 模型支持 dummy thought signature
@@ -370,7 +371,7 @@ func buildParts(content json.RawMessage, toolIDToName map[string]string, allowDu
 			// signature 处理：
 			// - Claude 模型（allowDummyThought=false）：必须是上游返回的真实 signature（dummy 视为缺失）
 			// - Gemini 模型（allowDummyThought=true）：优先透传真实 signature，缺失时使用 dummy signature
-			if block.Signature != "" && (allowDummyThought || block.Signature != dummyThoughtSignature) {
+			if block.Signature != "" && (allowDummyThought || block.Signature != DummyThoughtSignature) {
 				part.ThoughtSignature = block.Signature
 			} else if !allowDummyThought {
 				// Claude 模型需要有效 signature；在缺失时降级为普通文本，并在上层禁用 thinking mode。
@@ -381,7 +382,7 @@ func buildParts(content json.RawMessage, toolIDToName map[string]string, allowDu
 				continue
 			} else {
 				// Gemini 模型使用 dummy signature
-				part.ThoughtSignature = dummyThoughtSignature
+				part.ThoughtSignature = DummyThoughtSignature
 			}
 			parts = append(parts, part)
 
@@ -411,10 +412,10 @@ func buildParts(content json.RawMessage, toolIDToName map[string]string, allowDu
 			// tool_use 的 signature 处理：
 			// - Claude 模型（allowDummyThought=false）：必须是上游返回的真实 signature（dummy 视为缺失）
 			// - Gemini 模型（allowDummyThought=true）：优先透传真实 signature，缺失时使用 dummy signature
-			if block.Signature != "" && (allowDummyThought || block.Signature != dummyThoughtSignature) {
+			if block.Signature != "" && (allowDummyThought || block.Signature != DummyThoughtSignature) {
 				part.ThoughtSignature = block.Signature
 			} else if allowDummyThought {
-				part.ThoughtSignature = dummyThoughtSignature
+				part.ThoughtSignature = DummyThoughtSignature
 			}
 			parts = append(parts, part)
 

backend/internal/pkg/antigravity/request_transformer_test.go

@@ -86,7 +86,7 @@ func TestBuildParts_ThinkingBlockWithoutSignature(t *testing.T) {
 				if len(parts) != 3 {
 					t.Fatalf("expected 3 parts, got %d", len(parts))
 				}
-				if !parts[1].Thought || parts[1].ThoughtSignature != dummyThoughtSignature {
+				if !parts[1].Thought || parts[1].ThoughtSignature != DummyThoughtSignature {
 					t.Fatalf("expected dummy thought signature, got thought=%v signature=%q",
 						parts[1].Thought, parts[1].ThoughtSignature)
 				}
@@ -126,8 +126,8 @@ func TestBuildParts_ToolUseSignatureHandling(t *testing.T) {
 		if len(parts) != 1 || parts[0].FunctionCall == nil {
 			t.Fatalf("expected 1 functionCall part, got %+v", parts)
 		}
-		if parts[0].ThoughtSignature != dummyThoughtSignature {
-			t.Fatalf("expected dummy tool signature %q, got %q", dummyThoughtSignature, parts[0].ThoughtSignature)
+		if parts[0].ThoughtSignature != DummyThoughtSignature {
+			t.Fatalf("expected dummy tool signature %q, got %q", DummyThoughtSignature, parts[0].ThoughtSignature)
 		}
 	})