feat(server): add stateless OpenAI Responses API

[Iwctwbh]

Summary

• add POST /v1/responses as a stateless compatibility layer over the existing chat dispatch path
• support non-streaming Responses JSON and typed SSE lifecycle events for text, reasoning summaries, and custom function calls
• support replay of message, reasoning-summary, function_call, and function_call_output input items
• preserve request metadata (instructions, tools, tool choice, token limit, temperature, top-p, text verbosity, metadata) in response objects
• report full input usage plus KV-cached tokens through input_tokens_details

Explicit capability boundaries

Unsupported semantics return an OpenAI-shaped 400 instead of being silently ignored:

• previous_response_id, conversation, and store=true
• truncation="auto"
• image/file content and opaque reasoning without a readable summary
• hosted tools such as web/file search
• structured JSON output formats
• parallel_tool_calls=false

The supported path is deliberately stateless: clients replay prior message and function-call items in input and use store=false.

Protocol compatibility

The SSE order and object shapes follow the official OpenAI Responses lifecycle for text and function calls:

• response.created / response.in_progress
• response.output_item.added
• content or function-arguments delta/done events
• response.output_item.done
• response.completed or response.incomplete

Reference: https://developers.openai.com/api/reference/typescript/resources/beta/subresources/responses/methods/create

I also checked the lifecycle against the local DeepSeek Harness Responses fixture at cd5ef81481; focused tests cover both its text and function-call flows.

Scope

Extracted from #6 and rebuilt on current main (3901476d). This PR contains no metrics endpoint and no ToolCallStart change.

Verification

• cargo check --workspace --all-targets
• cargo test -p infr-server -p infr-chat (94 + 58 unit tests, plus 5 integration tests)
• cargo fmt --check
• git diff --check

Clippy was not run because the current Rust toolchain does not have the Clippy component installed.