test(bdd): prove three-replica Stargate registration in the multi-cluster Helmfile flow

Why

The secure multi-cluster registration BDD is intended to prove that a Pylon registers with every LLM request-router replica. It asserts router identities 0, 1, and 2 and three registration/tunnel connections, but the shared local fixture currently configures addons.llm.requestRouter.replicaCount: 1.

Make the three-replica topology explicit so the live BDD proves the supported minimum replica configuration rather than relying on fixture defaults or fake wiring output.

Scope

Extend tests/bdd/features/multi-cluster-helmfile-llm-registration-tls.feature.

• Set addons.llm.requestRouter.replicaCount: 3 in the feature-specific control-plane environment values.
• Keep DNS discovery enabled for the multi-replica StatefulSet.
• After installation, use an operator-visible kubectl command to assert that llm-request-router has desired replicas 3 and ready replicas 3.
• Retain the trusted Watch assertion for router identities llm-request-router-0, -1, and -2.
• Retain the Pylon metrics assertions that prove three registration streams and three reverse tunnels.
• Retain the authenticated LLM invocation after registration.
• Update the focused Godog wiring test for the new StatefulSet observation.

Acceptance criteria

• The live feature explicitly configures three LLM request-router replicas.
• The control-plane StatefulSet reports three desired and three ready replicas.
• A compute-plane Pylon is observable as connected to all three replicas.
• An authenticated LLM request succeeds after all three registrations are established.
• The feature fails when the configured replica count or a router registration is missing.

Non-goals

• Do not change the shared local BDD fixture default replica count.
• Do not add duplicate three-replica lifecycle coverage to generic Helmfile install smoke tests.
• Do not test router failover or pod deletion in this issue.

Start with tests/bdd/features/multi-cluster-helmfile-llm-registration-tls.feature and the focused Godog wiring test. Set the feature-specific request-router replica count to 3, keep DNS discovery and the existing Watch, metrics, tunnel, and authenticated LLM assertions, and add the operator-visible StatefulSet check. Done means the live BDD observes three desired and ready replicas and fails if any registration or the final request is missing.