fix: accept an app whose broker is created in an on_startup hook

[lesnik512]

Closes #56. Option 3 from the issue.

What

setup_di raised Broker must be defined to setup DI when the app had no broker at call time. FastStream supports creating the broker inside an on_startup hook and attaching it with app.add_broker (its set_broker docstring names this as the intended use), and the guard rejected that app at import time.

How

• The guard in setup_di is gone. Since fix: install the DI middleware on every broker, on startup #52 the middleware is installed by a startup hook that reads app.brokers, so nothing at setup_di time depends on a broker existing.
• The check moves into that install hook. If app.brokers is still empty when it runs, it raises a RuntimeError naming both remedies: pass a broker to FastStream(...) or app.add_broker(...), and register a broker-adding on_startup hook before setup_di, because hooks run in registration order. This fires before FastStream's own assert self.brokers.
• The FromDI message-time error from fix: name setup_di and app startup when FromDI finds no request container #55 gains one sentence for the other order: a broker added by a hook registered after setup_di is not covered.
• README setup_di bullet and ADR-0002 updated. The ADR paragraph that deferred this decision now records it, including the one case that still surfaces at message time: a later hook adds a broker while another broker already exists.

Tests

Both written first and red on main with the old guard:

• test_broker_added_by_a_startup_hook_registered_before_setup_di_gets_di: FastStream() with no broker, an on_startup hook that calls add_broker, then setup_di; a FromDI subscriber on that broker resolves under TestApp.
• test_app_without_broker_at_startup_names_both_remedies: setup_di on a broker-less app succeeds, and app.start() raises the new error.

test_app_without_broker is replaced by these two, since setup_di no longer raises there. just lint-ci clean, just test-ci at 100% coverage.