Streaming reasoning events translation

[HXYerror]

Part of #1. Depends on #4, #9.

Goal

Translate the SSE event stream of upstream `/responses` (and the streamed `reasoning_content` chunks of upstream `/chat/completions`) into:

• OpenAI-shape SSE on `/v1/responses` (mostly straight passthrough)
• Anthropic-shape SSE on `/v1/messages` (`message_start` → `content_block_start` (thinking) → `thinking_delta` × N → `signature_delta` → `content_block_stop` → next content_block_start (text) → ...)

Background

Upstream Responses-API SSE events include:

• `response.created`, `response.in_progress`, `response.completed`, `response.failed`
• `response.output_item.added` / `.done`
• `response.content_part.added` / `.done`
• `response.output_text.delta` / `.done`
• `response.reasoning.delta` / `.done`
• `response.reasoning_summary_text.delta` / `.done`
• `response.function_call_arguments.delta` / `.done`

These need to map to Anthropic's:

• `message_start`, `content_block_start`, `content_block_delta` (with `text_delta` | `thinking_delta` | `signature_delta` | `input_json_delta`), `content_block_stop`, `message_delta`, `message_stop`, `ping`

Current state

`src/routes/messages/stream-translation.ts` only handles the chat-completions stream shape (single `choices[0].delta`). `thinking_delta` and `signature_delta` types exist in `anthropic-types.ts:145–146` but have no producer.

Tasks

• On `/v1/responses` streaming: passthrough upstream SSE with minimal transformation (only add headers / handle disconnects)
• On `/v1/messages` streaming when target model is `responses`-mode (Model-to-endpoint routing (chat vs responses) #5):
  • Emit Anthropic `message_start`
  • For each upstream `response.output_item.added` of `type: reasoning` → emit Anthropic `content_block_start` (thinking)
  • Map `response.reasoning_summary_text.delta` → `thinking_delta`
  • On reasoning item completion, emit `signature_delta` carrying `encrypted_content` then `content_block_stop`
  • For `message` items → `content_block_start` (text), `response.output_text.delta` → `text_delta`
  • For `function_call` items → `content_block_start` (tool_use), `response.function_call_arguments.delta` → `input_json_delta`
  • On `response.completed` → `message_delta` (with stop_reason and usage) + `message_stop`
• On `/v1/messages` streaming when target model is `chat`-mode and upstream returns `reasoning_content` deltas (Reasoning types & reasoning_effort passthrough in chat-completions #7): emit `thinking` block in addition to `text` block
• Periodic `ping` events to keep long reasoning sessions alive

Acceptance criteria

• `/v1/messages` streaming with `gpt-5.3-codex` shows incremental `thinking` text in Claude Code's UI
• Tool calls inside reasoning context preserve correct ordering (reasoning → tool_use → reasoning → text)
• Stream cancellation propagates upstream

File pointers

• `src/routes/messages/stream-translation.ts` (extend)
• New: `src/routes/responses/stream-translation.ts` (passthrough/normalization)

[HXYerror]

Implemented in commits 8804287, 47c5486.

What was built:

• src/routes/messages/responses-stream-translation.ts: Translates Responses API SSE events to Anthropic SSE format
  • response.created → message_start (with fallback id if upstream omits it)
  • response.output_item.added reasoning/message/function_call → content_block_start
  • response.reasoning_summary_text.delta → thinking_delta
  • response.output_item.done reasoning → signature_delta + content_block_stop
  • response.output_text.delta → text_delta; response.content_part.done → content_block_stop
  • response.function_call_arguments.delta → input_json_delta
  • response.completed → message_delta + message_stop; ping every 20 events
• src/routes/messages/handler.ts: Replaced 501 stub with real streaming path

Key invariants upheld:

• encrypted_content emitted as signature_delta before content_block_stop
• Parse failures never block stream forwarding
• message_start always emitted (even when upstream omits response.id)
• stop_reason never null in message_delta
• Double content_block_stop prevented for function_call items
• Unknown item types skip block allocation (no orphaned stops)

210 tests passing across 13 files, lint clean.