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_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)
 		}
 	})