Add permissionless rebate stake return

[mswilkison]

Summary

• add permissionless RebateStaking.returnStake(staker) after deprecation
• reuse the existing full-stake withdrawal cleanup path
• keep third-party returns constrained to staker -> staker so callers cannot redirect funds
• cover successful third-party return and unavailable return cases

Stack

• Stacked on Add protocol peg keeper allowlist #981 / codex/protocol-peg-keeper

Tests

• npm run test -- ./test/bridge/RebateStaking.test.ts --grep deprecate
• npm exec eslint -- test/bridge/RebateStaking.test.ts

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/return-stake-after-deprecation

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