Add model helpers for clearing and reloading cached relationships

[techmahedy]

Problem

Loaded relationships are cached on the model instance. If a many-to-many relationship is modified afterward, the model can continue returning stale data:

$post = Post::find(1);

$post->tags; // Loads and caches tags

$post->tags()->relate([1, 4]); // Updates the database

$post->tags; // Still returns the old cached tags

Solution

Added two methods to the base model:

$post->forget('tags');

Clears the cached relationship without executing a new query.

$post->reload('tags');

Clears the cached relationship and immediately loads the latest data from the database.

Example:

$post->tags()->relate([1, 4]);

$tags = $post->reload('tags');

Subsequent access also returns the refreshed relationship:

$post->tags; // Returns tags 1 and 4

Changes

1. Added Model::forget(string $relation)
2. Added Model::reload(string $relation)
3. Added regression coverage for stale relationship data after relate()
4. Preserved existing relationship caching behavior for normal access