[CT-08] Build DocumentRevocationValidator

[mftee]

Problem

The submit and transfer handlers do not check whether a document has already been revoked before processing the request. A revoked document can still be re-submitted or transferred, which creates invalid on-chain records.

Proposed Solution

Create a DocumentRevocationValidator inside contract/module/revocation-check/ as a shared guard function that submit and transfer handlers can call before processing.

Acceptance Criteria

• is_revoked(cache: &CacheBackend, document_hash: &str) async function checks for a key revoke:{document_hash} in Redis
• Returns true if the key exists (document is revoked), false if it does not
• The submit handler (CT-01) calls is_revoked and returns 409 with error document has been revoked when the result is true
• The transfer handler calls is_revoked and returns 409 with the same message
• is_revoked returns an error (not a false) when the Redis connection fails, so the calling handler can return 500 rather than silently allowing the operation
• Unit tests cover: not revoked (key absent), revoked (key present), Redis error case
• Implementation files live inside contract/module/revocation-check/

[mxllv]

@mxllv has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

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[drips-wave]

Congratulations, @mxllv! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

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