Add access control to Pausable trait examples

[leighmcculloch]

What

pause() and unpause() default implementations in the v25 contracttrait migration guide now call require_auth on an admin address retrieved from instance storage. A __constructor is added to each example to initialize the admin address. Inline string literals for storage keys are replaced with named constants ADMIN and PAUSED.

Why

The previous examples exported pause() and unpause() as publicly callable contract functions without any authorization check. Developers following the pattern as written would ship contracts with open pause entrypoints. This ensures the examples start from a secure baseline. Closes #1763.

Close #1763

[Copilot]

Pull request overview

Updates the v25 migration note for #[contracttrait] to demonstrate a more realistic “pausable” pattern that includes admin authorization, constructor initialization, and test setup using testutils mock auths.

Changes:

• Introduces an auxiliary (non-exported) RequireAuthForPause trait and makes Pausable depend on it.
• Adds an __constructor to initialize an admin value in instance storage and enforces require_auth() in pause/unpause.
• Expands the doctest examples to show MockAuth/MockAuthInvoke usage for authorized calls.