fix(sandbox-manager): resolve post-claim 502s and add peer reconciliation for split-brain recovery

[AiRanthem]

Summary

• Fix a regression that could return 502 after sandbox claiming by correcting proxy routing/ext-proc behavior and sandbox manager infra interaction.
• Add peer reconciliation capability to recover from split-brain scenarios and improve cluster peer consistency.

Changes in This PR

1) Fix 502 errors after sandbox claiming

• Update proxy request handling and route resolution logic:
  • pkg/proxy/ext_proc.go
  • pkg/proxy/routes.go
• Add/adjust regression tests for proxy behavior:
  • pkg/proxy/ext_proc_test.go
• Align sandbox manager infra behavior with updated routing expectations:
  • pkg/sandbox-manager/infra/sandboxcr/infra.go

2) Peer reconciliation for split-brain healing

• Introduce peer reconciliation implementation:
  • pkg/peers/reconcile.go
• Add dedicated tests for reconciliation behavior:
  • pkg/peers/reconcile_test.go
• Integrate reconciliation with existing peer/memberlist flow:
  • pkg/peers/memberlist.go
  • pkg/peers/peers.go
• Update related tests and wire-up points:
  • pkg/peers/memberlist_test.go
  • pkg/sandbox-gateway/server/server.go
  • pkg/sandbox-manager/core.go

Test Plan

• Updated proxy unit tests for post-claim routing/ext-proc behavior.
• Added peer reconciliation unit tests, including split-brain recovery paths.
• Run full upstream CI for integration-level verification.

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[kruise-bot]

@AiRanthem: PR needs rebase.

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