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- Suppress errcheck for xxhash Digest.Write (never returns error)
- Add enable_cch_signing field to settings API contract test

Fixes GO-2026-4947, GO-2026-4946, GO-2026-4870, GO-2026-4869,
GO-2026-4866, GO-2026-4865 in crypto/x509, crypto/tls, archive/tar,
and html/template.

- Sync cc_version in x-anthropic-billing-header with the fingerprint
  User-Agent version, preserving the message-derived suffix
- Implement xxHash64-based CCH signing to replace the cch=00000
  placeholder with a computed hash
- Add admin toggle (enable_cch_signing) under gateway forwarding settings,
  disabled by default

commit f3aa54b7 的 rewriteSystemForNonClaudeCode 未能通过 Anthropic 第三方检测，
根因是两个关键信号与真实 Claude Code 不一致：

1. anthropic-beta 头缺少 claude-code-20250219：伪装路径主动将该 beta
   加入 drop set 并移除，但 Anthropic 依赖此 beta 识别 Claude Code 请求。
   修复：非 haiku 模型的伪装请求强制包含 claude-code beta。

2. system 字段使用 string 格式而非 array+cache_control：真实 Claude Code
   始终以 [{type,text,cache_control:{type:"ephemeral"}}] 发送 system，
   string 格式成为第三方检测信号。
   修复：rewriteSystemForNonClaudeCode 改为注入 array 格式。

附带调整：stripSystemCacheControl 按 system 是否被重写动态决定，
重写时保留 CC prompt 的 cache_control，未重写时（haiku/已含CC前缀）
保持原有剥离行为。

当上游返回SSE格式响应(如sub2api链路)时，API Key账号的非流式路径
未检测SSE，导致终态事件中空output直接透传给客户端。

- 将Content-Type SSE检测从仅OAuth扩展至所有账号类型
- 重命名handleOAuthSSEToJSON为handleSSEToJSON（无OAuth专属逻辑）
- 为透传路径新增handlePassthroughSSEToJSON，支持SSE转JSON及空output重建
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

上游API近期更新后，response.completed终态SSE事件的output字段可能为空，
实际内容仅通过response.output_text.delta等增量事件下发。流式路径不受影响，
但chat_completions非流式路径和responses OAuth非流式路径只依赖终态事件的
output，导致返回空响应。

新增BufferedResponseAccumulator累积器，在SSE扫描过程中收集delta事件内容
（文本、function_call、reasoning），当终态output为空时补充重建。

同时修复handleChatBufferedStreamingResponse遗漏response.done事件类型的问题。

Anthropic近期引入基于system参数内容的第三方应用检测机制，原有的前置追加
Claude Code提示词策略无法通过检测（后续内容仍为非Claude Code格式触发429）。

新策略：对非Claude Code客户端的OAuth/SetupToken账号请求，将system字段
完整替换为Claude Code标识提示词，原始system内容作为user/assistant消息对
注入messages开头，模型仍接收完整指令。

仅影响/v1/messages路径，chat_completions和responses路径保持原有逻辑不变。
真正的Claude Code客户端请求完全不受影响（原样透传）。

refactor(channel): split long functions, extract shared validation, move billing validation to service

- Split Update (98→25 lines), buildCache (54→20 lines), Create (51→25 lines)
  into focused sub-functions: applyUpdateInput, checkGroupConflicts,
  fetchChannelData, populateChannelCache, storeErrorCache, getOldGroupIDs,
  invalidateAuthCacheForGroups
- Extract validateChannelConfig to eliminate duplicated validation calls
  between Create and Update
- Move validatePricingBillingMode from handler to service layer for
  proper separation of concerns
- Add error logging to IsModelRestricted (was silently swallowing errors)
- Add 12 new tests: ToUsageFields, billing mode validation, antigravity
  wildcard mapping isolation, Create/Update mapping conflict integration

Antigravity groups were incorrectly matching pricing and model mapping
entries from anthropic/gemini platform tabs. Each platform should be
strictly isolated — antigravity groups only use antigravity-tagged pricing.

Restore gateway_service.go, setting_handler.go, routes/admin.go,
dto/settings.go, group_repo.go, api_key_repo.go, wire_gen.go to
upstream/main versions and surgically remove only Sora references.

This preserves upstream-only features (RequireOauthOnly, RequirePrivacySet,
GroupResolution, etc.) that were missing when using release branch versions.

When a channel has no model mapping for the requested model, ChannelMappedModel
equals OriginalModel (the user's arbitrary input). Combined with the default
BillingModelSource="channel_mapped", this incorrectly overrides the BillingModel
set by the OpenAI format conversion layer (e.g., gpt-5.4 from DefaultMappedModel)
back to the unmapped original model (e.g., glm) which has no pricing — resulting
in zero-cost billing.

Add guard condition so the channel_mapped override only fires when the channel
actually changed the model (ChannelMappedModel != OriginalModel).

- Remove media_type column from all INSERT/SELECT/SCAN in usage_log_repo
- Remove media_type mock arg from request_type and integration tests
- Adjust scan stub value arrays from 47 to 46 elements

- gofmt: user.go, config_test.go, group_handler.go, smart_retry_test.go
- Remove unused: mergeGroupIDs, resolveProxyURL, "time" import
- Fix api_contract_test.go: remove extra Sora args from NewAdminService,
  NewSettingHandler, NewAccountHandler; remove Sora field expectations
- Fix account_test_service_openai_test.go: restore test helpers

- Run gofmt on user schema, config test, group handler
- Remove unused mergeGroupIDs function
- Restore shared test helpers (newJSONResponse, queuedHTTPUpstream)
  that were in deleted Sora test file

- applyRequestTierOverrides now uses filterValidIntervals consistently
  with applyTokenOverrides (per_request/image modes were not filtering)
- CostInput accepts optional pre-resolved pricing via Resolved field,
  eliminating duplicate Resolver.Resolve() calls in gateway billing paths

- Remove unused claudeMax*Tokens constants (Claude Max feature not included)
- Remove unused UsageMapHook type, SetUsageMapHook method, and usageToMap function
- Fix gofmt formatting in channel_service.go, openai_model_mapping_test.go,
  chatcompletions_to_responses.go

Restore account_usage_service.go, antigravity_gateway_service.go,
antigravity_credits_overages.go and its test to upstream/main state.
These credits balance precheck changes were accidentally included
during cherry-pick of channel management commits.