fix: avoid HTTP probes on gRPC worker groups

[mikemikimike]

Summary

• avoid generating the model-based HTTP probe for non-router replica groups
• keep the default probe for router groups and preserve explicitly configured probes
• add a regression test for router/worker probe selection

Tests

• uv run pytest src/tests/_internal/server/services/jobs/configurators/test_service.py::test_default_probe_only_for_router_replica_group -q
• uv run pytest src/tests -q (354 passed, 4891 skipped)
• uv run ruff check .
• uv run ruff format --check .
• uv run pyright -p .
• git diff --check