chore(deps): grpc 4→5, protobuf 4→6

Two coupled majors that have been blocking the rest of the
Dart-side dependency lift in Studio (the constraint solver
pins meta/characters/etc. through these). Picked up:

- grpc 5.1.0 (was 4.2.0)
- protobuf 6.0.0 (was 4.2.0)
- analyzer 13.0.0, _fe_analyzer_shared 100.0.0
- google_cloud 0.5.0, googleapis_auth 2.3.1 (transitive)

The bindings now live in `lib/src/generated/fai/v1/` only —
the well-known-types `lib/src/generated/google/protobuf/`
copy has been deleted. protobuf 6 ships its own bundled
`well_known_types/google/protobuf/empty.pb.dart` and the
`HubAdminClient` API expects the *packaged* Empty type;
emitting a local copy created two distinct Empty types and
broke every `_admin.<rpc>(Empty())` call site.

`tools/generate.sh` now skips the WKT generation pass and
wipes `lib/src/generated/google/` on every run so the
duplicate-types breakage can't regress silently. The
import in `hub_client.dart` switches from the local copy to
`package:protobuf/well_known_types/google/protobuf/empty.pb
.dart`.

Smoke-verified: dart analyze clean, dart test green
(4 tests). Bumped to 0.17.0 — Studio absorbs in the next
commit.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/src/hub_client.dart

@@ -14,7 +14,8 @@ import 'channel_factory_io.dart'
 import 'generated/fai/v1/common.pb.dart' as pb_common;
 import 'generated/fai/v1/hub.pb.dart' as pb;
 import 'generated/fai/v1/hub.pbgrpc.dart' as grpc_hub;
-import 'generated/google/protobuf/empty.pb.dart' show Empty;
+import 'package:protobuf/well_known_types/google/protobuf/empty.pb.dart'
+    show Empty;
 
 /// Connection coordinates for a hub endpoint.
 class HubEndpoint {