fix: validate network and document state edge cases

[Mkalbani]

Summary

Fixes edge-case validation for network configuration, ownership chains, revocation, and transfers.

Changes

• [CT-81] network module has no test for a testnet/mainnet configuration mismatch #1205 / CT-81: Validate that the configured Horizon URL matches STELLAR_NETWORK.
• [CT-80] ownership_chain module has no test for a broken or circular chain #1204 / CT-80: Detect broken and circular ownership chains.
• [CT-78] POST /revoke has no test for revoking an already-revoked or unknown hash #1202 / CT-78: Return distinct errors for unknown and already-revoked hashes.
• [CT-79] POST /transfer has no test for transferring an already-revoked document #1203 / CT-79: Reject transfers for revoked documents.

Testing

• Added focused Rust tests for all four cases.
• cargo fmt --check
• cargo test

Issues

Closes #1205
Closes #1204
Closes #1202
Closes #1203

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[mftee]

No merge conflicts with main. Good handling of the network and document state edge cases - approving.