feat(csharp): support nested container field codec

[chaokunyang]

Why?

C# generated serializers need field-level schema descriptors that can describe the exact Fory wire type for scalars and nested containers. The previous [Field(Encoding = ...)] model only handled a narrow integer-encoding override, which was not expressive enough for nested annotated container metadata or compatible-mode skipping based on remote field metadata.

What does this PR do?

• Replaces the C# [Field] / FieldEncoding API with [ForyField], supporting optional stable field ids and Type = typeof(S...) schema descriptors.
• Adds Apache.Fory.Schema.Types descriptor marker types for scalar, packed-array, list, set, and map field metadata.
• Extends the C# source generator to emit descriptor-driven field codecs for scalar, list, and map payloads, including nested generics and nullability-aware generated read/write helpers.
• Updates compatible field skipping so C# consumes payloads according to remote nested field metadata, including scalar, packed array, list/set, and map descriptors.
• Updates the Fory compiler C# generator to emit [ForyField(Type = typeof(...))] hints, including nested map/list descriptors and reduced-precision Half / BFloat16 carriers.
• Adds C# runtime, compiler-generator, and xlang peer coverage for unsigned schema descriptors and nested annotated containers, and moves the nested annotated container xlang checks to CSharpXlangTest.
• Refreshes the C# README and field-configuration guide for the new [ForyField] descriptor API.

Related issues

#1017
#3630
#3625
#3630
#3636

AI Contribution Checklist

• Substantial AI assistance was used in this PR: yes / no
• If yes, I included a completed AI Contribution Checklist in this PR description and the required AI Usage Disclosure.
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Does this PR introduce any user-facing change?

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?

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