Add DocumentJsonConverter for System.Text.Json serialization support

[AlexDaines]

Description

Add DocumentJsonConverter to fix System.Text.Json serialization of Document types. Document implements IEnumerable interfaces which causes STJ to treat it as a collection, throwing InvalidDocumentTypeConversionException for non-list/non-dictionary types.

Also fixes Document.FromObject number parsing to try TryGetInt32 before TryGetInt64, consistent with DocumentUnmarshaller.

Motivation and Context

Fixes #3694, #3837, #4078.

Testing

• 10 test methods covering primitives, collections, round-trip, regression, and customer scenario reproductions.
• dotnet test --filter "FullyQualifiedName~Document" → 26 passed, 0 failed.
• Dry run: DRY_RUN-0a4d1a6e-09a4-4bf0-ae36-79e79bba4de8
• Status: Running

Breaking Changes Assessment

No breaking changes. DocumentJsonConverter is internal sealed. The [JsonConverter] attribute changes STJ behavior from broken (throws) to correct. Customer workarounds (custom converters in JsonSerializerOptions) take precedence per STJ resolution order.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license

[Copilot]

Pull request overview

This PR fixes System.Text.Json serialization/deserialization for Amazon.Runtime.Documents.Document by adding a dedicated STJ converter so Document is no longer treated as an IEnumerable collection during serialization, and aligns Document.FromObject numeric parsing behavior with existing runtime unmarshalling logic.

Changes:

• Added an internal DocumentJsonConverter and applied it to Document via [JsonConverter].
• Updated Document.FromObject to prefer TryGetInt32 before TryGetInt64.
• Added unit tests covering primitive/collection serialization, round-trips, and regressions; added a patch DevConfig entry.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
sdk/src/Core/Amazon.Runtime/Documents/DocumentJsonConverter.cs	Implements STJ converter to serialize/deserialize Document by its DocumentType rather than via IEnumerable.
sdk/src/Core/Amazon.Runtime/Documents/Document.cs	Applies the converter attribute and updates FromObject number parsing to try int before long.
sdk/test/UnitTests/Custom/Runtime/Documents/DocumentJsonConverterTests.cs	Adds coverage for the new converter, including regressions and customer scenarios.
generator/.DevConfigs/180bd698-bd6a-4799-a23e-db90f89660d6.json	Declares a patch-level core changelog entry for the fix.

[normj]

Very cool PR. This will help make working with JSON and Documents.