Replace BaseHTTPMiddleware subclass with a pure-ASGI implementation.
BaseHTTPMiddleware wraps responses in _StreamingResponse and preserves
the original Content-Length, so streamed bodies (JSONResponse with
ensure_ascii=False, client disconnects mid-response, CORS+Prometheus+
RequestId chains) trigger:

    RuntimeError: Response content longer than Content-Length

Public API is preserved: import path, app.add_middleware signature,
HEADER_NAME constant, request.state.request_id, request_id_ctx and the
echoed x-request-id response header all behave as before.

BREAKING: subclasses overriding `dispatch` will break — the method no
longer exists. No such consumers found in Lucente.

Refs: encode/starlette#1678