Switch `mpc-contract` to Promise + callback verification

[pbeza]

Background

With dcap-qvl-free wire types now in place (#3235, #3245, #3246), mpc-contract can finally call out to tee-verifier.near over a cross-contract Promise instead of running dcap_qvl::verify::verify itself. This is where the WASM size drop materializes.

User Story

As an MPC contract maintainer, I want submit_participant_info to verify Dstack attestations via cross-contract Promise to tee-verifier.near, so that dcap-qvl and its closure leave the MPC contract's WASM and the contract gains safe headroom under NEP-509's 1,490,000-byte limit.

Acceptance Criteria

• Attestation::verify splits into extract_dcap_inputs (pre-Promise) + finish_verify (in callback).
• submit_participant_info's Dstack arm schedules a Promise to tee-verifier.{near,testnet}'s verify_quote and stashes a PendingAttestation keyed by account_id.
• Mock attestations remain synchronous (no Promise, no pending entry).
• New on_attestation_verified callback re-reads allowlists fresh from tee_state, runs the post-DCAP body, inserts into stored_attestations, refunds the caller's deposit on failure (or on PromiseError::Failed), and clears the pending entry.
• State migration initializes the new pending_attestations map empty.
• cargo tree -p mpc-contract --target wasm32-unknown-unknown -e no-proc-macro -i dcap-qvl returns nothing in normal (non-dev) deps.
• Reproducible WASM drops at least 200 KB below the 1,490,000-byte NEP-509 limit.
• Unit tests cover both submit_participant_info arms and the four callback paths (success, promise failed, post-DCAP failed, allowlist re-read).

Resources & Additional Notes

PR: #3247. Depends on #3235, #3237, #3245, #3246. Tracked by #3264.

[netrome]

Nit - I prefer if we keep the acceptance criteria written concisely. Ideally just one or two sentences. These long checklists that Claude often generates tend to look more like a brief implementation plan than a description of what the issue aims to achieve.

I'd suggest something like the following instead:

Acceptance Criteria

submit_participant_info delegates attestation verification to a dedicated attestation verification contract.