feat(runtime): add stream_sse / stream_sse_sync for SSE endpoints

[calvin-archastro]

What changed

Adds the SSE primitives the generated stream() methods target for x-sdk-streaming endpoints:

• HttpClient.stream_sse (async) — async def ... -> AsyncIterator[dict]
• SyncHttpClient.stream_sse_sync (sync) — def ... -> Iterator[dict]

Each opens the request with any method + JSON body, sets Accept: text/event-stream, raises ApiError on a non-2xx response before the stream opens, and yields parsed {"event", "data"} records from the text/event-stream body via httpx streaming (aiter_lines / iter_lines).

classDiagram
    class HttpClient {
      request(path, ...) Any
      request_raw(path, ...) dict
      stream_sse(path, ...) AsyncIterator
    }
    class SyncHttpClient {
      request(path, ...) Any
      request_raw(path, ...) dict
      stream_sse_sync(path, ...) Iterator
    }
    note for HttpClient "stream_sse: method + body, SSE parse, yields {event, data}"

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Scope

Runtime-only. No codegen changes — this is the hand-maintained runtime the generated SSE methods call. (Generator-side emission + contract tests live in archastro-openapi.)

Risk

Low. Purely additive — new methods on the existing clients; nothing else changes. One known limitation: token refresh is not retried mid-stream (a 401 raises ApiError); noted for a follow-up if needed.

User impact

Generated SSE stream() methods (async + sync) now have a working runtime to call.

Testing

Added unit tests (tests/test_http_client.py, 42 pass) mirroring the TS suite: async + sync ordered events + JSON body send + Accept header, and non-2xx → ApiError. ruff check + ruff format clean.

Follow-ups

• Mid-stream 401 refresh (deferred — minimal version raises instead).
• Codegen that emits the stream() methods + contract tests is tracked separately (archastro-openapi).