feat(refund-vault): add oracle aggregator + median-based dynamic refund policy engine

[adelekevictor12]

Closes #133

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[mergekeeper]

MergeKeeper review

Scope: in scope for linked issue #133.
Verdict: clean

The PR successfully implements the oracle aggregator, standard oracle interface, and median-based dynamic refund policy engine with thorough test coverage and security considerations.

Reviewed commit: 988c658ae516aed215472ae3ec811258831d10b4.
CI and merge eligibility are checked separately.

[mergekeeper]

Approved

Successfully designed and integrated the decentralized oracle aggregator and dynamic refund policy engine with comprehensive test coverage.

[mergekeeper]

MergeKeeper merge status

Status: blocked
PR state: open
Mergeability: mergeable
Checked commit: 988c658ae516aed215472ae3ec811258831d10b4.

Reason: One or more required CI checks failed.

Failing checks:

• build-wasm - view details
• clippy - view details
• test - view details
• fmt - view details

Next steps:

1. Open the failing check details above and fix the reported error.
2. Run the same checks locally where possible.
3. Commit and push the fix.
4. MergeKeeper will automatically re-review the updated PR.

[adelekevictor12]

done