test: Add invalid signature test with wrong admin public key

[0xtrendz]

Overview

This PR adds a regression test for the admin signature validation path. Initialization with one admin public key must reject signatures produced by a different private key. The test initializes the contract with pubkey A, signs a mint attempt with key B, asserts the mint attempt fails, and verifies that balance and latest period remain untouched.

Related Issue

Changes

🔐 Invalid Signature Test

• [ADD] src/test.rs
  • Adds test_invalid_admin_signature covering the wrong-admin-key rejection path.
  • Initializes the contract with admin pubkey A.
  • Signs the mint attempt with key B (derived from a different private key).
  • Asserts the mint attempt fails.
  • Verifies the failure path leaves balance and latest period unchanged.

Verification Results

cargo test
✅ 1/1 passed (test_invalid_admin_signature)

Live acceptance check:
✅ Initialized with pubkey A, signed with key B
✅ Mint attempt fails
✅ Balance and latest period remain unchanged

Acceptance Criteria	Status
Test initializes with pubkey A and signs with key B	✅ test_invalid_admin_signature initializes with pubkey A and signs with key B
The mint attempt fails	✅ Mint attempt is asserted to fail
Failure path leaves balance and latest period untouched	✅ Balance and latest period asserted unchanged after the failed mint

Closes #234

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