[CT-79] POST /transfer has no test for transferring an already-revoked document

[mftee]

Overview

contract/src/handlers/transfer.rs records ownership transfer (CT-03), but there's no evident guard preventing a transfer on a document whose anchor was already revoked — a revoked document shouldn't be transferable, but nothing currently tests or enforces that ordering.

Tasks

• Add a check that rejects a transfer request for a revoked document's hash
• Add a test asserting a transfer attempt after revocation is rejected with a clear error

Acceptance Criteria

• Attempting to transfer a revoked document's hash is rejected, and this is tested

Notes for Contributors

Comment below to be assigned.