Tests for Responses path & reasoning fidelity

[HXYerror]

Part of #1. Final-mile work to make sure the new path is regression-safe.

Goal

Add unit and integration-style tests covering the new /v1/responses route, the Anthropic→Responses adapter, reasoning round-trip, and streaming event translation.

Current state

tests/ contains:

• anthropic-request.test.ts
• anthropic-response.test.ts
• create-chat-completions.test.ts

No tests touch responses, gpt-5, reasoning, codex, or encrypted_content.

Tasks

• tests/responses-request.test.ts — Anthropic Messages → Responses input items
  • System → instructions
  • Tools translation
  • Tool result blocks → function_call_output items
  • Thinking config → reasoning effort tier (verify the 10k/5k/2k thresholds match the spec in Reasoning types & reasoning_effort passthrough in chat-completions #7)
  • Round-trip of an Anthropic thinking block with signature into a reasoning item with encrypted_content (Model-to-endpoint routing (chat vs responses) #5, Anthropic /v1/messages → Responses API adapter #8)
• tests/responses-response.test.ts — Responses output → both OpenAI and Anthropic
  • reasoning → thinking block with signature
  • message → text block
  • function_call → tool_use
  • encrypted_content preserved byte-exact
• tests/responses-stream.test.ts — SSE event sequencing
  • Reasoning-then-text
  • Text-then-tool-then-text
  • Tool-then-text-then-reasoning (out-of-order index handling)
  • Stream error mid-flight produces a clean message_stop
• tests/model-routing.test.ts — verify /v1/chat/completions for gpt-5.3-codex either bridges or returns a clear 400 (per the policy in Add upstream Responses API service client #4)
• Add a fixture file under tests/fixtures/responses/ with realistic upstream SSE traces (capture once with --show-token --verbose and scrub)

Acceptance criteria

• bun test passes locally
• Coverage for the new src/routes/responses/, src/routes/messages/responses-*translation.ts, and src/services/copilot/create-responses.ts modules is ≥ 80%
• Existing tests remain green

File pointers

• tests/anthropic-request.test.ts — reference style
• tests/anthropic-response.test.ts — reference style
• AGENTS.md — test conventions

[HXYerror]

Duplicate of the corresponding #13 — closing to avoid confusion.