🐛 Fix response_class=EventSourceResponse sending an empty body for non-generator endpoints - #16192

See #16107

A path operation declared with response_class=EventSourceResponse that is not a generator is dispatched into the SSE branch anyway, because get_request_handler() recomputes is_sse_stream from the response class alone, with no generator check.

@app.get("/events", response_class=EventSourceResponse)
async def events() -> EventSourceResponse:
    return EventSourceResponse(gen())
# returns 200 text/event-stream with an empty body

The branch is entered before the endpoint runs, so dependant.call(...) yields a coroutine (or a plain value) where an iterator is expected, and the 200 and text/event-stream headers are already flushed by the time it fails. Two symptoms, both silent:

• async def returns 200 with an empty body, plus RuntimeWarning: coroutine ... was never awaited. Under uvicorn it is worse than empty: iterate_in_threadpool() raises TypeError: 'coroutine' object is not iterable mid-response, the connection truncates, and the client gets RemoteProtocolError: peer closed connection without sending complete message body.
• sync def returns 200 streaming the iteration of the return value with no error at all, so an endpoint returning {"msg": "hello"} puts data: "msg" on the wire.

The same value is already computed correctly on the route, with the generator check fused in, right beside its JSONL sibling:

route.is_sse_stream = is_generator and lenient_issubclass(
    response_class, EventSourceResponse
)
route.is_json_stream = is_generator and isinstance(response_class, DefaultPlaceholder)

but only is_json_stream is passed down, so the two disagree for every non-generator endpoint.

This PR removes the local recomputation and threads route.is_sse_stream through as a parameter, exactly as route.is_json_stream already was, so both flags are derived in one place. The local recomputation appears to have guarded against response_class being a DefaultPlaceholder, but add_api_route() already resolves that via get_value_or_default() before the route is built, so the route attribute is correct in precisely that case — verified for app-level and router-level default_response_class=EventSourceResponse, where route.is_sse_stream stays True and OpenAPI still documents text/event-stream.

Present since SSE was added in 0.135.0; verified against stock 0.135.0 and 0.141.0.

Adds tests/test_sse_non_generator.py covering:

• a non-generator returning an EventSourceResponse directly
• an async non-generator returning a plain value
• a sync non-generator returning a plain value (the silent-corruption case)
• a generator, as the existing control case for normal SSE streaming

Three of the four fail on master — two with TypeError: 'coroutine' object is not iterable, and the sync one with a plain assertion showing data: "msg" on the wire.