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Update the default user-agent version from 1.18.4 to 1.19.6
to match the latest official antigravity client.

PR #635 returned HTTP 200 with {"input_tokens": 0} when upstream doesn't
support count_tokens (404). This caused Claude Code CLI to trust the zero
value, believing context uses 0 tokens, so auto-compression never triggers.

Fix: return 404 with proper error body so CLI falls back to its local
tokenizer for accurate estimation. Return nil (not error) to avoid
polluting ops error metrics with expected 404s.

Affected paths:
- Passthrough APIKey accounts: upstream 404 now passed through as 404
- Antigravity accounts: same fix (was also returning fake 200)

FetchGoogleOneTier 原先在方法内部直接创建 DriveClient 实例，
导致单元测试中对 googleapis.com 发起真实 HTTP 请求，在 CI 环境
产生 401 错误。

将 DriveClient 作为依赖注入到 GeminiOAuthService，遵循项目
端口与适配器架构规范：
- 新增 repository/gemini_drive_client.go 作为 Provider
- 注册到 repository Wire ProviderSet
- 测试中使用 mockDriveClient 替代真实调用

第三方 Anthropic 中转站通常不支持 /v1/messages/count_tokens 端点，
上游返回 404 时降级返回 {input_tokens: 0}，客户端 fallback 到本地估算。

- 仅匹配 404 状态码，语义明确：端点不存在
- 其他错误 (400/429/500) 保留原始处理链和 ops 遥测
- 无需解析错误消息内容，不依赖字符串匹配
- 新增 table-driven 测试覆盖 fallback 和 non-fallback 路径

使用安全的 comma-ok 模式替代裸类型断言，避免 golangci-lint errcheck 报错。
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

新增 jsonMarshalRaw 使用 SetEscapeHTML(false) 替代 json.Marshal，
避免 HTML 字符转义导致客户端无法直接使用返回的 URL。
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

使用 go-sora2api v1.1.0 SDK 替代原有 ~2000 行自建 HTTP/PoW/TLS 指纹代码，
SDK 提供高并发性能优化（实例级 rand、PoW 缓冲区复用、context.Context 支持）。

- 新增 SoraSDKClient 适配器实现 SoraClient 接口
- 精简 sora_client.go 为仅保留接口和类型定义
- 更新 Wire 绑定使用 SoraSDKClient
- 删除 SoraDirectClient、sora_curl_cffi_sidecar、sora_request_guard 等旧代码
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

- Remove Gemini platform exclusion from model restriction UI in
  Create/Edit account modals (Gemini now supports model_mapping)
- Remove outdated Gemini model passthrough info cards
- Add model_mapping field to GeminiCredentials type
- Extend warmup request interception toggle to Antigravity platform
- Remove redundant try/catch in API key account creation
- Remove noisy gateway.request_completed debug log
- Reorganize Gemini model mapping sections in constants.go

The ClientSecret constant was left as an empty string after
getClientSecret() was refactored to use defaultClientSecret.
Remove the dead constant and update the test accordingly.

- Fix oauth_test.go and client_test.go to use defaultClientSecret
  variable instead of env var (init() already sets the default)
- Align migration 058 gemini-3-pro-high/low/preview mappings with
  constants.go (map to 3.1 versions)

Add a built-in default for ANTIGRAVITY_OAUTH_CLIENT_SECRET so the
service works out of the box without requiring environment variable
configuration. The env var can still override the default.

Add missing gemini-3.1-pro-preview -> gemini-3.1-pro-high mapping to
DefaultAntigravityModelMapping for consistency with migration 059.

Update the default ANTIGRAVITY_USER_AGENT_VERSION from 1.84.2 to
1.18.4 to match the current Antigravity-Manager desktop client.

Add claude-sonnet-4-6 to identity injection modelInfoMap and
Antigravity model selector. Add gemini-3.1-pro-high/low to
Antigravity model list and Sonnet 4.6 preset mapping.

- Add claude-sonnet-4-6 to default Antigravity model mapping
- Add antigravity_extra_retries default value in config
- Add cache-dependency-path to CI setup-go for faster builds
- Simplify vitest config to avoid vite plugin compatibility issues

Add comprehensive tests for warmup request interception behavior
covering Antigravity accounts with various credential configurations.

Remove the special case that bypassed model-supported checks for Gemini
API Key accounts, allowing model_mapping to filter requests properly.
Add tests for multiplatform model filtering behavior.

- Use mapped model (billingModel) instead of original request model for billing
- Use resolveFinalAntigravityModelKey for 429 rate limit model key,
  ensuring rate limit records match the actual upstream model
- Add regression tests for both fixes

- Extract duplicated failover logic from gateway_handler.go (3 places)
  and gemini_v1beta_handler.go into shared failover_loop.go
- Introduce FailoverState with HandleFailoverError and HandleSelectionExhausted
- Move helper functions (needForceCacheBilling, sleepWithContext) into failover_loop.go
- Add comprehensive unit tests (32+ test cases)
- Delete redundant gateway_handler_single_account_retry_test.go

Add a dedicated CheckMixedChannel endpoint that allows the frontend
to pre-validate mixed channel risk before submitting create/update
requests. This improves UX by showing warnings earlier in the flow
instead of only after form submission.

Backend changes:
- Add CheckMixedChannelRequest struct and CheckMixedChannel handler
- Register POST /check-mixed-channel route
- Expose CheckMixedChannelRisk as public method on AdminService
- Simplify Create/Update 409 responses (remove details/require_confirmation)
- Add comprehensive handler tests and stub methods

Frontend changes:
- Add checkMixedChannelRisk API function and TypeScript types
- Refactor CreateAccountModal to precheck before step transition and submission
- Refactor EditAccountModal to precheck before update submission
- Replace pendingPayload pattern with action-based dialog flow

Extract applyInterceptWarmup utility to unify all credential building
call sites:
- Fix upstream account creation missing intercept_warmup_requests write
- Fix apikey edit mode missing else-branch to clear the setting
- Add backend unit test for IsInterceptWarmupEnabled
- Add frontend unit test for credentialsBuilder

Before this change, when a client disconnected mid-request, the error
message was "Upstream request failed after retries", which is misleading
and pollutes error logs. Now we check context.Err() to return a more
accurate "Client disconnected" message for both Claude and Gemini
forward paths.

skipIfExternalServiceUnavailable 检查了 "timeout" 但 Go 的
context.DeadlineExceeded 错误信息是 "context deadline exceeded"，
不包含 "timeout" 子串，导致外部服务不可达时测试直接失败而非跳过。