refactor(sse): declare the ArrayBuffer backing on media byte producers

[backryun]

Part of the TS7 readiness campaign (#8484). One root cause, 5 diagnostics, zero new.

Root cause

Six declarations spell a byte buffer as bare Buffer / Uint8Array. Without its type argument that widens to ArrayBufferLike, which also admits SharedArrayBuffer — so the value is rejected at every Web API boundary it is actually passed to: BodyInit for new Response(...), BlobPart for new Blob([...]).

Every one of them is already ArrayBuffer-backed at runtime:

producer	allocation	consumed by
hexToBytes() (audioSpeech.ts)	new Uint8Array(len)	new Response(bytes) — MiniMax T2A v2
synthesizeGtts() (gtts.ts)	Buffer.concat(...)	new Response(audio) — gTTS
pcmToWav() → vertexGenerateSpeech() (vertexMedia.ts)	Buffer.concat([header, pcm])	new Response(audio) — Vertex Gemini TTS
fetchRemoteImage() (remoteImageFetch.ts)	Buffer.from(await response.arrayBuffer())	new Blob([...]) — Topaz upload
readPageResponseBody() (browserPool.ts)	Buffer.from(body) (copies)	browserBackedChat.ts response body

The declarations were simply less specific than the values, so this states what the code already guarantees. It is the same fix #8533 applied to the multipart and gRPC-web bodies.

Diagnostics fixed

file	count	code
open-sse/handlers/audioSpeech.ts	3	TS2345 → BodyInit
open-sse/handlers/imageGeneration.ts	1	TS2322 → BlobPart
open-sse/services/browserBackedChat.ts	1	TS2322 → Buffer<ArrayBuffer>

Measured as a line-number-agnostic diff of the complete tsc -p open-sse/tsconfig.json error set against a freshly re-measured base: 208 → 203, 5 fixed, 0 new.

Note that three of the five fixed diagnostics are in files this PR does not touch — the fix is at each producer, not at the call site.

Behaviour

Type annotations only; no runtime change. The narrowing is safe in both directions: Buffer<ArrayBuffer> is assignable to Buffer, so no existing consumer is affected, and every producer's allocation is ArrayBuffer-backed (Node buffers are never SharedArrayBuffer-backed, pooled or not).

Test plan

New tests/unit/media-body-arraybuffer-backing.test.ts — 4 tests, one per reachable producer. Each asserts both the backing and that the value really is accepted by the Web API it is passed to in production:

assert.ok(wav.buffer instanceof ArrayBuffer);
assert.doesNotThrow(() => new Response(wav));

The existing tests for these functions assert their contents — the RIFF header (vertex-media.test.ts), the concatenated chunks (gtts-provider.test.ts), the decoded base64 (remote-image-fetch.test.ts). None asserted the backing, which is the one property the narrowed return types exist to guarantee and the one a refactor to a shared allocation would break.

The sixth producer, hexToBytes(), is module-private; its new Response(bytes) path is already exercised end-to-end by minimax-tts-1043.test.ts, which builds the response and reads its body.

node --import tsx/esm --test tests/unit/media-body-arraybuffer-backing.test.ts   # 4/4

Regression run over every touched producer's existing suite — gtts-provider, vertex-media, vertex-tts, remote-image-fetch, remote-image-fetch-dns-rebinding, remote-image-fetch-pin-dns-connection, browser-pool-optional-import, minimax-tts-1043, audio-speech-handler, audio-speech-fishaudio, image-generation-handler: 108 pass, 0 fail.

Gates: typecheck:core clean, check:file-size OK, ESLint clean on all touched files.

The one red check, Merge integrity (changelog + generated skills), is base-red on release/v3.8.49 and unrelated to this diff — it reproduces on the bare tip and is being fixed by #8657 / tracked in #8658.