fix(xai): map x_search, fill tool descriptions, and isolate protocol cache

[ChenYCL]

Summary

Four local overlay fixes that belong on official master:

1. Grok x_search → web_search
   Grok Responses emit x_search / x_search_call. Codex/OpenAI clients only accept web_search, web_search_2025_08_26, function, mcp, file_search, computer_use_preview, code_interpreter. Rewrite those types on the client-bound xAI OAuth Codex stream so serde does not fail.

2. GPT-5 max_completion_tokens
   Anthropic → OpenAI Chat Completions still copied max_tokens. Azure GPT-5.x / o-series reject it. Convert gpt-5* / o1* / o3* / o4* to max_completion_tokens, and retry once on unsupported_parameter + max_tokens.

3. Protocol capability per session
   Auto protocol learning is keyed by session (Claude Code session/thread, OpenCode session, Responses execution, then token hash). One desktop session that probed Codex on Grok no longer pins another session.

4. Missing Claude tools[].description
   Grok's CLI proxy requires the key. Fill from name (or type) when the field is absent. Existing descriptions stay.

Test

go test -tags sonic ./internal/app -run 'TestRewriteXAIResponsesToolTypesForClient|TestWrapXAIClientToolTypeRewriteSSE|TestOpenAIRetryBodyForMaxTokens|TestProtocolCapabilityCacheIsSessionScoped|TestProtocolCapabilitySessionIDFromHeaders|TestProtocolCapabilityCacheClearChannelsKeepsOtherChannels'
go test -tags sonic ./internal/protocol/cliproxy/util -run TestEnsureClaudeToolDescription
go test -tags sonic ./internal/protocol/cliproxy/openai/claude -run TestConvertClaudeRequestToOpenAI_GPT5UsesMaxCompletionTokens

[caidaoli]

Thanks for this — all four fixes landed in ac80274, reworked at their root causes (you're credited as co-author):

1. x_search → web_search: rewritten only when the client did not declare x_search itself; SSE event lines follow the payload type.
2. max_completion_tokens: renamed before send for gpt-5/o-series OpenAI Chat requests (before channel BodyRules), plus a one-shot retry when the upstream explicitly rejects max_tokens. Kept in the app wire layer; the cliproxy snapshot stays untouched.
3. Protocol cache: a fallback after a local translation miss no longer overwrites the learned protocol, instead of introducing a per-session cache.
4. Tool descriptions: filled only for xAI hosts (cli-chat-proxy.grok.com / api.x.ai); official Anthropic server tools reject extra fields.

Not ported: hiding internal x_search custom_tool_call traces (CLIProxyAPI f0f13cfd). Closing in favor of that commit.