feat: Improve Pausable Migration Guide and Add Migration Tests

[r-near]

Purpose

Enhances migration documentation and adds tests for migrating from single manager_roles to separate pause_roles/unpause_roles in the Pausable plugin.

Changes

1. Updated Migration Guide:

   • Added warnings about preserving enum variant order in #[derive(AccessControlRole)]
   • Emphasized adding new roles at enum end to maintain permission bitflag mapping
   • Provided migration code examples for both separation and backward compatibility

2. Added Migration Tests:

   • Created test contracts demonstrating old/new implementations
   • Implemented test verifying permission behavior through upgrade process
   • Demonstrated safe role addition with maintained permissions

Testing

• Added test contracts (pausable_old, pausable_new)
• Implemented pausable_migration.rs test that:
  • Validates both pre/post-migration behaviors
  • Verifies proper permission inheritance
  • Tests role separation after migration

[kiseln]

Feeling dejavu like I already reviewed this