[CONTRACT] Canonical signature_payload encoding for session-key authorization

[wheval]

Feature Description

Define and enforce a single canonical byte encoding for signature_payload so the same intent cannot be re-signed under different encodings (malleability / replay class).

Problem Statement

Open #936: no canonical encoding requirement. SDK, relayer, and contract must agree on bytes-to-sign.

Proposed Solution

1. Spec in docs/contract-methods or CAP-style note.
2. Contract verifies exact encoding; SDK helpers build only that form.
3. Vectors in account-abstraction + contract tests.
4. Relayer verify uses same helper.

Impact Area

• Smart Contracts
• SDK
• Security

Priority

• Critical

Implementation Complexity

• Complex

Related

#936

[OZILSOLAR]

Hi team!!!!
I would like to work on this issue!!!!!

[Okorie2000-code]

Hi Maintainer,

I'd like to work on this issue. I have experience with smart contracts, SDK development, and security-focused systems. I can submit a clean PR within 48 hours.

[nurudeenheroic]

Hi! I can handle this issue and ensure it meets the requirements.

[Nemenwa]

Hello maintainer, I have reviewed the task description and will like to work on this issue please. ETA: 6hours