[12.x] Add missing @throws annotations to Encrypter class

[sumaiazaman]

This PR adds missing @throws annotations to the Encrypter class methods that throw exceptions but were missing proper documentation.

Changes Made

• ensureTagIsValid() method: Added @throws \Illuminate\Contracts\Encryption\DecryptException annotation
• previousKeys() method: Added @throws \RuntimeException annotation

Why This Matters

These methods throw exceptions in their implementation but were missing the proper @throws annotations in their docblocks. This improvement:

• Enhances IDE support and autocomplete functionality
• Improves static analysis capabilities (PHPStan, Psalm, etc.)
• Provides better documentation for developers using these methods
• Maintains consistency with Laravel's documentation standards

Testing

No functional changes were made - only documentation improvements. The existing test suite should continue to pass without modification.