Fix SSE/JSONL stream_item_type lost through include_router - #15426

Fixes #15401.

Problem

APIRoute.__init__ only detects streaming return annotations inside the
isinstance(response_model, DefaultPlaceholder) branch. include_router
calls add_api_route with the already-resolved response_model (None
for streaming routes), so the detection is skipped on the merged route.
Both stream_item_type and stream_item_field stay None.

Consequences:

• OpenAPI schema drops contentSchema / itemSchema for the route.
• The request handler, which is built in __init__ via
  request_response(self.get_route_handler()), closes over
  stream_item_field=None, so runtime validation and serialization of
  stream items are silently disabled.

Fix

In APIRouter.include_router, right after add_api_route appends the
merged route, copy stream_item_type and stream_item_field from the
source route, then rebuild new_route.app so the handler closure picks
up the restored field. An isinstance(new_route, APIRoute) check
narrows Starlette's BaseRoute for mypy.

Both SSE (EventSourceResponse) and JSONL routes go through the same
path, so both are covered.

Tests

Four tests in tests/test_sse.py:

• test_stream_item_type_preserved_through_include_router asserts both
  attributes are set after include_router.
• test_sse_stream_item_field_set_on_direct_route baseline for
  directly-registered routes.
• test_sse_openapi_schema_after_include_router verifies the item
  model appears in components.schemas.
• test_sse_runtime_serialization_after_include_router end-to-end
  HTTP call via TestClient checking the serialized frame body.

Full suite passes locally. ruff check, ruff format --check, and
mypy fastapi/routing.py are clean.