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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。

- 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

- Add int64(0) param to SelectAccountWithLoadAwareness callers (signature change from channel scheduling refactor)
- Add UsageMapHook type and struct field to StreamingProcessor
- Revert Claude Max cache billing code to upstream/main (not part of channel feature)
- Revert credits overages logic to upstream/main (non-channel change)
- Remove Instructions field reference (non-channel OpenAI feature)
- Restore sora_client_handler_test.go from upstream + add channel service nil params

- Fix errcheck: defer rows.Close() with nolint
- Fix errcheck: type assertion with ok check in channel cache
- Fix staticcheck ST1005: lowercase error string
- Fix staticcheck SA5011: nil check cost before use in openai gateway
- Fix gofmt: format chatcompletions_to_responses.go

Add reverse-direction converters for Anthropic platform groups to accept
OpenAI-format requests:

- ResponsesToAnthropicRequest: Responses API input → Anthropic Messages
  request with system extraction, tool/toolChoice mapping, reasoning
  effort conversion, image data URI↔base64, and consecutive role merging
- AnthropicToResponsesResponse: Anthropic response → Responses response
  with content block→output item mapping, usage, stop_reason→status
- AnthropicEventToResponsesEvents: stateful SSE stream converter
  (Anthropic streaming protocol → Responses streaming protocol)
- FinalizeAnthropicResponsesStream: synthetic termination for
  incomplete streams

旧映射错误地将所有级别上移一档（medium→high, high→xhigh），
导致 effort=max 被原样透传到 OpenAI 上游并返回 400 错误。

根据两边官方 API 定义对齐：
- Anthropic: low, medium, high（默认）, max
- OpenAI:    low, medium, high（默认）, xhigh

新的 1:1 映射：low→low, medium→medium, high→high, max→xhigh

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

当 Claude Code 发来的 MCP tool 的 input_schema 为 {"type":"object"} 且缺少
properties 字段时，OpenAI Codex 后端会拒绝并报错：
Invalid schema for function '...': object schema missing properties.

新增 normalizeToolParameters 函数，在 convertAnthropicToolsToResponses 中
对每个 tool 的 InputSchema 做规范化处理后再赋给 Parameters。
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

- apply default mapped model only when scheduling fallback is actually used

- preserve reasoning in OpenAI-compatible output via reasoning_content and avoid invalid input function_call ids

将 /v1/chat/completions 端点从 ResponseWriter 劫持模式重构为独立的
类型安全转换路径，与 Anthropic Messages 端点架构对齐：

- 在 apicompat 包新增 Chat Completions 完整类型定义和双向转换器
- 新增 ForwardAsChatCompletions service 方法，走 Responses API 上游
- Handler 改为独立的账号选择/failover 循环，不再劫持 Responses handler
- 提取 handleCompatErrorResponse 为 Chat Completions 和 Messages 共用
- 删除旧的 forwardChatCompletions 直传路径及相关死代码

Align struct field assignments and fix indentation detected by
golangci-lint v2.9's gofmt checker.

Add Anthropic Messages API support for OpenAI platform groups, enabling
clients using Claude-style /v1/messages format to access OpenAI accounts
through automatic protocol conversion.

- Add apicompat package with type definitions and bidirectional converters
  (Anthropic ↔ Chat, Chat ↔ Responses, Anthropic ↔

 Responses)
- Implement /v1/messages endpoint for OpenAI gateway with streaming support
- Add model mapping UI for OpenAI OAuth accounts (whitelist + mapping modes)
- Support prompt caching fields and codex OAuth transforms
- Fix tool call ID conversion for Responses API (fc_ prefix)
- Ensure function_call_output has non-empty output field
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>