fix(serialization): defer interface collection codec dependencies

[ReubenBond]

Fixes #10300

Orleans 10.2 interface collection codecs eagerly unwrapped their fallback element codecs during construction. A generated type containing an interface collection field could therefore fail while its codec was being constructed—even when the field was null—because the interface codec resolved an unavailable element codec before any runtime collection value existed.

Keep the injected fallback codec holders lazy so concrete runtime codecs are resolved first and null fields require no element codec. Preserve the existing interface codec type as the fallback wire-format identity, and cover both generated null-field and custom concrete-runtime-codec scenarios.

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[Copilot]

Pull request overview

This PR addresses a regression introduced in Orleans 10.2 where interface collection codecs eagerly resolve element codecs, causing IList<SequentialItem<T>> codec resolution to fail because SequentialItem<T> was not marked as serializable.

Changes:

• Mark SequentialItem<T> as a generated-serializable type with stable member IDs.
• Add a regression test which validates codec resolution for SequentialItem<T> and IList<SequentialItem<T>>, plus a simple round-trip serialization check.

Show a summary per file

File	Description
test/Orleans.Core.Tests/Serialization/BuiltInSerializerTests.cs	Adds regression coverage for SequentialItem<T> and interface-list codec resolution plus a round-trip check.
src/Orleans.Streaming/Core/IAsyncBatchObserver.cs	Annotates SequentialItem<T> with [GenerateSerializer] and stable [Id] attributes so codecs can be generated/resolved.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 1

[Copilot]

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 1

[Copilot]

Copilot's findings

Comments suppressed due to low confidence (1)

test/Orleans.Serialization.UnitTests/InterfaceCollectionCodecTests.cs:549

• class declarations in C# require a body; public sealed class UnsupportedElement; will not compile. Use an empty body instead.

    public sealed class UnsupportedElement;

• Files reviewed: 2/2 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

Comments suppressed due to low confidence (2)

src/Orleans.Streaming/Core/IAsyncBatchObserver.cs:28

• SequentialItem<T>.Item no longer has an [Id] attribute. This changes the serialized schema and will break compatibility with previously serialized payloads and existing generated-code expectations for this type.

        /// <value>The item.</value>
        public T Item { get; }

src/Orleans.Streaming/Core/IAsyncBatchObserver.cs:35

• SequentialItem<T>.Token no longer has an [Id] attribute. This changes the serialized schema and will break compatibility with previously serialized payloads and existing generated-code expectations for this type.

        public StreamSequenceToken Token { get; }
    }

• Files reviewed: 3/3 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0 new