feat(typespec-ts): generate structured JSONL and SSE streaming by default

[l0lawrence]

Summary

Adds structured JSONL + SSE streaming code generation to the @azure-tools/typespec-ts modular emitter. A response whose TCGC metadata indicates a structured stream now generates a Promise<AsyncIterable<T>> of deserialized items/events instead of returning the raw binary Uint8Array body. This mirrors microsoft/typespec#11594 (Python emitter) and the sibling @typespec/http-client-js work.

This is the default behavior — there is no opt-in option.

What's generated

• JSONL (JsonlStream<T>) → Promise<AsyncIterable<T>> that lazily decodes JSON Lines.
• SSE (SSEStream<T>) → Promise<AsyncIterable<...>> that dispatches each Server-Sent Event by its event: name (via @azure/core-sse), deserializes the payload, and stops at the terminal event. Full heterogeneous multi-event dispatch and @terminalEvent handling are supported (both named terminals, keyed on the terminal flag, and unnamed data: [DONE] sentinel terminals).

Operations connect eagerly (HTTP status/errors surface at call time) and decode the body lazily.

Implementation

• New static helper static/static-helpers/streamingHelpers.ts (getStreamResponse, readJsonlStream, readSseStream, SseEventDescriptor), registered in static-helpers-metadata.ts and loaded only when the package actually has a structured-stream operation (packageHasStructuredStreaming, a side-effect-free metadata check).
• operation-helpers.ts: getStructuredStreamInfo resolves stream metadata (gated purely on response.streamMetadata / sseMetadata); branches in the operation and deserialize builders emit the AsyncIterable shape.
• @azure/core-sse added to AzureCoreDependencies and injected into the generated package's dependencies only when an SSE op is present.

Tests

Three new unit-modular scenarios: JSONL, SSE named (heterogeneous + terminal [DONE]), and SSE unnamed (homogeneous message events). Terminal handling was verified against the real http-specs SSE spec wire frames plus a runtime smoke of the helper.

Validation

• pnpm build (tsc) — pass
• pnpm lint (oxlint --deny-warnings) — pass
• unit-modular — 662 passed
• test-next — 254 passed
• SSE runtime smoke against exact spec frames — pass

Only the 3 new streaming scenarios are semantically changed baselines; no non-streaming snapshots change.

[l0lawrence]

Superseded by the upstream draft PR against Azure/typespec-azure main (Azure#5196). Closing this fork-targeted duplicate; branch and commit remain intact.