feat(serialization): add codecs for FrozenDictionary and FrozenSet

[ReubenBond]

Problem

Orleans had no serialization support for the frozen collection types introduced in .NET 8 (FrozenDictionary<TKey, TValue> and FrozenSet<T>). Attempting to serialize a grain call, storage payload, or stream event containing one of these types would fail.

Solution

Add serialization codecs and copiers for FrozenDictionary<TKey, TValue> and FrozenSet<T>, modeled on the existing immutable-collection codecs. They are guarded by #if NET8_0_OR_GREATER so they only compile on platforms that ship the frozen collections (the netstandard2.1 target excludes them).

Notable details:

• Surrogates carry the entries as a List<> plus the collection's comparer (stored only when non-default). Reading entries straight out of the source collection avoids re-hashing keys when constructing the surrogate.
• Copiers deep-copy their contents in the same manner as the immutable-collection copiers, short-circuiting to a shallow copy when the element copiers are shallow-copyable.
• Frozen collection types are abstract base types whose runtime instances are internal subtypes, potentially of a different generic arity. The copiers therefore implement IDerivedTypeCopier, and CodecProvider.CreateCopierInstance now advances the field type and search type in lockstep when resolving derived-type copiers, mirroring the existing codec-side logic. Without this, resolving a copier for a subtype with mismatched arity threw an ArgumentException from MakeGenericType.

Tests cover round-tripping through the codec, copier behavior (including mutable element types), and comparer preservation.

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Pull request overview

Adds built-in Orleans.Serialization support for .NET 8 frozen collections so FrozenDictionary<TKey, TValue> and FrozenSet<T> can be serialized/copied in grain calls, storage payloads, and stream events on NET8+.

Changes:

• Add new codecs, surrogates, and deep copiers for FrozenDictionary<TKey, TValue> and FrozenSet<T> (NET8+ only).
• Update copier resolution in CodecProvider to correctly handle derived types whose generic arity differs from their base type (needed for frozen collections’ internal runtime types).
• Add unit tests covering round-trip serialization and copier behavior, including comparer preservation.

Show a summary per file

File	Description
test/Orleans.Serialization.UnitTests/BuiltInCodecTests.cs	Adds codec and copier tests for frozen collections and validates comparer preservation.
src/Orleans.Serialization/Serializers/CodecProvider.cs	Fixes derived-type copier lookup to advance field/search types together when walking base types.
src/Orleans.Serialization/Codecs/FrozenSetCodec.cs	Introduces NET8+ codec/surrogate/copier for FrozenSet<T>.
src/Orleans.Serialization/Codecs/FrozenDictionaryCodec.cs	Introduces NET8+ codec/surrogate/copier for FrozenDictionary<TKey, TValue>.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 2