Delegate: add intent verification

[altafan]

At the moment, the delegate verifies only the forfeit tx submitted by the client to prevent the future batch session from failing. It must also verify the client's intent, or if that's invalid, it won't be able to join the batch session at all

[arkana-ai-bot]

Arkana debug analysis 🤖

Classification: bug

Code location: internal/core/application/delegator_service.go:178 — newDelegateTask

The newDelegateTask function calls validateForfeits (which checks forfeit tx structure and signatures) but never validates the intent itself:

// delegator_service.go ~line 186
if err := validateForfeits(s.svc.publicKey, cfg, forfeitTxs); err != nil {
    return nil, err
}
// ← no corresponding validateIntent call

What's checked today:

• Forfeit tx inputs reference outpoints in the intent ✅
• Input VTXOs exist in the indexer and are not spent ✅
• Forfeit tx structure and delegate fee output ✅

What's not checked:

• The intent's outputs — whether destinations, amounts, and scripts are protocol-valid
• The intent's proof signature — whether the proof is correctly signed by the client's key
• Whether the intent would be accepted by arkd (VTXO constraints, scripts, weight limits, etc.)

Impact: A malformed or invalid intent submitted to the delegate will pass local validation, get saved to DB, and be registered via RegisterIntent on arkd. If arkd then rejects it during the batch session, the delegate has already committed and the batch fails — exactly the scenario described in the issue.

Suggested fix: Before saving the task, validate the intent against the same rules arkd enforces. At minimum:

1. Verify the proof signature (the client's pubkey signs the intent message)
2. Optionally do a dry-run SubmitTx / ValidateIntent RPC call to arkd before accepting the delegation

The forfeit validation pattern in validateForfeits can serve as a template for a new validateIntent function.

[Tyagiquamar]

Hi @aruokhai, I'd like to work on this if available. I checked internal/core/application/delegate_service.go:182 and confirmed newDelegateTask runs validateForfeits but doesn't verify the intent proof/message. I plan to add the missing verification and unit tests. Could you assign this to me?

[aruokhai]

@altafan , is this issue still a priority.