Add package-specific exception hierarchy

[trm42]

What

Introduces a small, package-owned exception hierarchy under the new Trm42\CacheDecorator\Exceptions namespace so consumers can catch every cache-decorator-specific failure with a single catch block, while still distinguishing the two concrete failure modes.

• CacheDecoratorException — abstract class … extends \Exception. Base type for every package error; callers catch this.
• MissingDecoratedObjectException extends CacheDecoratorException — replaces the LogicException in initDecorated() (no instance passed and decoratedClass() returned null).
• UndefinedMethodException extends CacheDecoratorException — replaces the BadMethodCallException raised for a forwarded call to a method that exists nowhere on the decorated object.

Why

The package previously threw raw SPL exceptions (LogicException, BadMethodCallException), which made it impossible to catch cache-decorator-specific errors as a group and coupled error handling to generic classes any dependency might also throw.

How

• initDecorated() now throws MissingDecoratedObjectException (message unchanged).
• Undefined forwarded calls now surface as UndefinedMethodException. Since calls are forwarded through Laravel's ForwardsCalls trait, this is done by overriding the trait's throwBadMethodCallException() helper — the single point that raised BadMethodCallException — keeping the message (Call to undefined method {Decorator}::{method}()) verbatim so only the thrown type changes.
• New PSR-4-mapped files under src/Exceptions/ (no composer change needed).
• Tests updated to assert the concrete subclasses; README documents the new namespace, the base type, when each child is thrown, and a catch (CacheDecoratorException $e) snippet.

Breaking change

The thrown types now extend \Exception via CacheDecoratorException and are not instances of LogicException / BadMethodCallException. Any downstream catch relying on those SPL types must be updated. Worth a release-notes/CHANGELOG line.

Verification

• composer check green: 31 tests pass, PHPStan reports no errors, Pint passes.

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