feat: Eloquent casts metadata cache

[binaryfire]

The existing per-instance getCasts() cache already avoids rebuilding the merged casts array on repeated calls. This PR builds on that by caching the derived cast metadata predicates that still re-resolved the same answers over and over for the same instance. Methods like isJsonCastable(), isEncryptedCastable(), isClassCastable(), isEnumCastable(), and getCastType() are called repeatedly during attribute reads/writes, toArray(), dirty checks, and original-value comparisons.

This change adds a single $castMetadataCache bucket on each model instance and memoizes those derived answers by predicate name and attribute key.

The cache uses one bucket property instead of one property per predicate. That keeps the per-model memory cost lower while still making each cached result explicit:

$this->castMetadataCache['jsonCastable'][$key]
$this->castMetadataCache['classCastable'][$key]
$this->castMetadataCache['castType'][$key]

Cached Paths

This PR caches:

• getCastType()
• isDateCastable()
• isDateCastableWithCustomFormat()
• isJsonCastable()
• isEncryptedCastable()
• isClassCastable()
• isEnumCastable()
• isClassDeviable()
• isClassSerializable()
• isClassComparable()

The invalid class-cast exception path isn't cached. Invalid casts still throw every time.

Invalidation

The new cache is cleared anywhere the existing merged-casts cache was already cleared:

• initializeHasAttributes()
• mergeCasts()
• setKeyName()
• setKeyType()
• setIncrementing()
• initializeSoftDeletes()
• __sleep()

Those call a shared flushCastCaches() helper so future cast metadata caches only need to be added in one place.

Background

This is in the same general problem discussed in Laravel's Eloquent casting performance thread:
laravel/framework#57101. That discussion shows how much time can be spent repeatedly resolving cast metadata on model-heavy workloads.

Tests

Added coverage for:

• invalidation after each cast mutation/reset path
• per-instance isolation
• withCasts() leakage prevention
• every cached predicate writing to its expected cache bucket
• invalid class casts continuing to throw on repeated calls