fix: echo back the model name the client sent

[Priyanshu-u07]

Request {"model": "vllm-14b"} came back as "model": "Qwen/Qwen2.5-14B-Instruct-AWQ". Every response path that carries a model field now echoes the name the client used.

Non-streaming

One shared helper, echo_requested_model in worker_routing.py, alongside upstream_model which already owns the send-side naming. All four handlers call it: chat completions, embeddings, images and video. It is a no-op for engines whose response has no model field, so images and video are covered without assuming what a custom diffusion engine returns.

The client's name is still available at that point — the pipeline only ever substitutes the upstream id into the outgoing body, never onto ctx.model.

Streaming

This needed more than an overwrite. Upstream is read with aiter_raw, so chunks are arbitrary byte fragments: a single data: line, or the model name inside it, can arrive split across two chunks, and a chunk cannot be rewritten on its own.

process_stream now re-frames into whole lines when a rewrite is requested, holding any trailing fragment for the next chunk and flushing at the end. It also decodes incrementally, because a per-chunk decode(errors="ignore") throws away both halves of a multibyte character that straddles a boundary and deletes it from the response. A 3-byte Devanagari character split after its first byte disappeared entirely before that fix.

Only events actually being rewritten are re-serialised. Error payloads, [DONE], blank separators and non-JSON lines are returned byte for byte. With no rewrite requested the original yield chunk path runs unchanged, so other callers are unaffected, and the usage-parsing buffer is untouched.

Tests

25 tests. The ones that matter are the boundary cases: a line split across chunks, a split inside the model name, one byte at a time, a multibyte character split mid-sequence, and a final line with no trailing newline.

Each fix is mutation-verified — a naive byte replace instead of line framing passes 11 tests and fails exactly the 3 boundary ones; the old per-chunk decode fails the multibyte test; removing the helper call fails that path's echo test.

Closes #353