[WIP] Gate indexer readiness on control plane

[Mallets]

Summary

• Gate indexer node readiness on control-plane client connectivity in addition to the existing metastore and ingester checks.
• Enable generated control-plane check_connectivity/endpoints methods so readiness uses the same endpoint-availability signal as metastore clients.
• Add unit and integration coverage proving indexers stay unready without a control-plane endpoint and return /health/readyz 503 after control-plane shutdown while metastore remains ready.

generate_extra_service_methods implications

• Adds generated check_connectivity() and endpoints() helpers to ControlPlaneService clients and implementations; it does not add protobuf RPCs or change wire compatibility.
• Remote clients consider the control plane available when their BalanceChannel contains at least one discovered endpoint. This is an endpoint-availability check, not an end-to-end RPC health probe.
• Local mailbox clients use actor connection state, while Tower-wrapped clients forward the helpers to the underlying client.
• Generated mocks expose the helpers, and any future handwritten ControlPlaneService implementation must implement them.

Test plan

• cargo check -p quickwit-proto
• cargo check -p quickwit-serve
• cargo test -p quickwit-serve test_readiness_updates --lib
• cargo test -p quickwit-integration-tests test_indexer_readiness_decreases_after_control_plane_shutdown
• cargo clippy -p quickwit-serve -p quickwit-integration-tests --all-features --tests
• make fmt