fix(apicompat): repair codex Responses<->Anthropic/ChatCompletions conversion

[hanyunsushi]

Problem

When a Responses API client (e.g. codex) is routed through an Anthropic upstream or a chat/completions-only upstream, requests fail or render blank due to several bugs in the apicompat conversion layer. Account "test connection" passes because it uses a self-constructed simple request that never exercises the Responses input parsing, masking these issues.

Related: #2913

Root causes & fixes

Request direction (ResponsesInputItem)

1. Arguments/Output typed as string — codex sends function_call.arguments as a JSON object and function_call_output.output as an array. The strict string type caused json.Unmarshal to fail (502 on the Anthropic path) or rawString to silently drop the content (chat/completions path). Both are now json.RawMessage with normalization helpers (normalizeResponsesArguments, extractResponsesOutputText, responsesArgumentsToChatString); reverse-direction builders wrap strings with jsonRawString to preserve the OpenAI wire format.

2. Tools without parameters produced a null input_schema — codex namespace tools (MCP/agent) and bare web_search carry no parameters; Anthropic 422s on a null schema. Now backfilled with an empty object schema.

3. web_search emitted as the Anthropic server tool web_search_20250305 — some Anthropic-compatible upstreams don't implement server tools (422). Now emitted as a plain function tool.

4. Top-level instructions and developer-role items were dropped — codex puts its primary system prompt in instructions; the old code only read system-role items, leaving the model with no instructions and (worse) leaking developer input_text blocks into a user message. Both now map to the Anthropic system field.

5. Empty/whitespace system + string-form system — an empty system 422s; and a string-form system combined with tools 422s on some upstreams. System is now omitted when empty and emitted in array form ([{"type":"text","text":...}]), which is spec-valid and what the official Claude Code client uses.

6. reasoning_effort: "xhigh" — codex sends xhigh for top-tier models; chat/completions upstreams only accept low/medium/high and 400 on xhigh. Now normalized to high.

Response direction (streaming → Responses events)

7. Missing response.content_part.added/.done — strict clients (codex) need a content part before output_text.delta; without it text deltas have nothing to attach to and the client renders nothing. Now emitted around the text block on both conversion paths.

8. output_item.done lacked content/tool-call fields — codex collects the final output and tool calls from OutputItemDone items, not from delta events. Message items now carry their full content, and function_call items carry call_id/name/arguments.

9. Ghost empty deltas — some upstreams send a leading {"content":""} chunk; emitting it produced an empty output_text.delta and a premature message item, leaving codex stuck on "thinking" with no output. Empty content/reasoning deltas are now skipped.

Tests

Adds coverage for all of the above: polymorphic arguments/output (string/object/array), tool schema backfill, web_search as function tool, instructions/developer → system, empty-system omission, xhigh normalization, content_part event ordering, output_item content/tool-call fields, and empty-delta skipping. All existing apicompat tests pass; gofmt/go vet clean.

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[hanyunsushi]

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