[Blazor] Make _serializerCache static readonly to avoid creating dictionaries per instance

[Copilot]

This change optimizes the PersistentStateValueProvider class by making the _serializerCache field static instead of instance-level.

Problem

Previously, every instance of PersistentStateValueProvider would create its own ConcurrentDictionary for caching serializers, even though the serializers are typically registered as singletons in the DI container and don't vary between instances.

Solution

Changed _serializerCache from private readonly to private static readonly. This allows all instances to share the same cache, reducing memory allocation and improving performance when multiple PersistentStateValueProvider instances are created.

Impact

• Performance: Reduces memory allocation by sharing the serializer cache across all instances
• Compatibility: No breaking changes - the cache behavior remains functionally identical
• Thread Safety: ConcurrentDictionary is already thread-safe, making the static sharing safe

Testing

• ✅ All Components tests pass (1,124 tests)
• ✅ All PersistentStateValueProvider tests pass (35 tests)
• ✅ All IPersistentComponentStateSerializer tests pass (2 tests)
• ✅ No build regressions in the core Components project

Fixes #62659.

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[ilonatommy]

If the CI is green.