feat: ship chats.stream_chat() with SSE support

[rodnnnney]

Summary

stream_chat had the same gap as the TypeScript SDK: the overlay mapped it, the docs documented it, but Speakeasy silently skipped the operation during codegen — the registry spec labels the stream response application/x-ndjson, and JSONL/NDJSON stream generation (jsonlResponses) is gated to Speakeasy's enterprise tier. SSE generation works on the current tier.

The endpoint is served by demo2's platform v2 handler (TextQLLabs/demo2#13014), which emits Server-Sent Events of {result: Cell} / {error: Status} envelopes.

Changes

• Overlay: maps the operation to chats.streamChat (name-mapping mirrored from the TS SDK) and normalizes the stream response to the SSE envelope contract — robust against any current registry snapshot; becomes a harmless no-op once demo2 publishes the SSE spec at the source.
• Regenerated SDK: ships chats.stream_chat() / stream_chat_async() returning EventStream[StreamChatEvent] with typed data.result: Cell / data.error: Status.

Usage

res = textql.chats.stream_chat(chat_id=chat_id, body={"latest_complete_cell_id": cell_id})
with res as event_stream:
    for event in event_stream:
        if event.data.result:
            ...  # next cell

Cross-repo context

• TextQLLabs/demo2#13014 — serves the endpoint as SSE natively and publishes the contract in the OpenAPI source.
• feat: ship chats.streamChat() with SSE support textql-ts-sdk#7 — same fix for the TypeScript SDK.

Merge order: this can merge independently (the overlay normalizes the old spec), but the endpoint only speaks SSE once the demo2 PR deploys — until then the generated method targets the new contract.