Clarify CastsAttributes::set() phpdoc to accept TGet and recommend symmetric casts

- Update @param type to `TGet|TSet|null` to reflect that `set()` may receive the runtime cast value (TGet) as well as raw DB-compatible value (TSet).
- Add note on assignment symmetry to avoid surprises when `$a = $a = $b` is used (common with caching).
- Recommend symmetric casts (TGet == TSet) where possible for safer and more predictable behavior.
- No runtime behavior changes.

Author: andreas-pms

src/Illuminate/Contracts/Database/Eloquent/CastsAttributes.php

@@ -5,6 +5,10 @@
 use Illuminate\Database\Eloquent\Model;
 
 /**
+ * Recommendation:
+ * Prefer symmetric casts (TGet == TSet) so "$a = $a = $b"
+ * remain safe and unsurprising.
+ * 
  * @template TGet
  * @template TSet
  */
@@ -24,10 +28,12 @@ public function get(Model $model, string $key, mixed $value, array $attributes);
     /**
      * Transform the attribute to its underlying model values.
      *
-     * @param  \Illuminate\Database\Eloquent\Model  $model
-     * @param  string  $key
-     * @param  TSet|null  $value
-     * @param  array<string, mixed>  $attributes
+     * Important:
+     * Implementations must tolerate receiving the runtime cast value (TGet) here.
+     * As with object caching, `set()` may receive the value returned by `get()` (TGet).
+     * Implementations should therefore accept both TGet and TSet.
+     *
+     * @param  TGet|TSet|null  $value
      * @return mixed
      */
     public function set(Model $model, string $key, mixed $value, array $attributes);