refactor(supervisor): remove legacy supervisor control RPCs after session migration

Problem Statement

#1731 should keep the existing supervisor control RPCs during the session-control migration so rollout is reversible and mixed-version gateways/supervisors remain safe. Once the session path is implemented, feature-flagged, and exercised, the redundant legacy RPCs should be deprecated or removed in a separate cleanup.

Proposed Design

After the session-control path is stable:

• Migrate supervisor call sites away from legacy polling/report RPCs.
• Remove or deprecate redundant RPCs where the session-control equivalent is adopted:
  • GetSandboxConfig
  • GetSandboxProviderEnvironment
  • PushSandboxLogs
  • ReportPolicyStatus
  • SubmitPolicyAnalysis
  • Inference.GetInferenceBundle
• Remove compatibility fallback code only after the supported rollout window is clear.
• Update generated clients, server handlers, tests, and docs together.
• Keep user-facing read APIs, such as log retrieval or policy status reads, if they are still independently useful.

Alternatives Considered

• Remove legacy RPCs inside #1731: rejected because it makes the first migration harder to review, harder to roll back, and riskier for mixed-version deployments.
• Keep legacy RPCs indefinitely: acceptable during rollout, but it leaves duplicate control paths and makes future behavior harder to reason about.

Agent Investigation

Current code still uses separate RPCs for the control/reporting paths while ConnectSupervisor only carries session lifecycle, heartbeat, and relay lifecycle messages. The initial #1731 implementation should make legacy RPC handlers thin wrappers around shared helpers, then route session messages through the same helpers. This follow-up removes the wrappers and fallback paths after the new path is proven.

Definition of Done

• Confirm the session-control path is enabled and covered by tests/e2e for supported deployments.
• Remove or deprecate redundant proto RPCs and regenerate bindings.
• Remove server handlers that no longer have supported callers.
• Remove supervisor polling/reporting call sites that no longer need fallback.
• Update docs that mention legacy polling/reporting behavior.
• Add release/compatibility notes if this is a breaking protocol cleanup.

Related

• Follow-up to #1731