[release/11.0-preview7] Add contentEncoding: base64 to JSON schema for byte[], Memory, and ReadOnlyMemory

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main PR #130881

Description

JsonSchemaExporter was omitting "contentEncoding": "base64" for byte-array types despite STJ serializing them as base64 strings. Per JSON Schema 2020-12 §8.3, contentEncoding must be set when the string payload is encoded.

• JsonSchema.cs: Added ContentEncodingPropertyName constant and ContentEncoding property; wired into KeywordCount and ToJsonNode
• ByteArrayConverter, MemoryByteConverter, ReadOnlyMemoryByteConverter: GetSchema() now returns ContentEncoding = "base64"
• JsonSchemaExporterTests.TestTypes.cs: Updated expected schemas for byte[], Memory<byte>, ReadOnlyMemory<byte>

Before / After:

// byte[] — before
{ "type": ["string", "null"] }
// byte[] — after
{ "type": ["string", "null"], "contentEncoding": "base64" }

// Memory<byte> / ReadOnlyMemory<byte> — after
{ "type": "string", "contentEncoding": "base64" }

Customer Impact

Consumers using JsonSchemaExporter to generate schemas for types with byte[], Memory<byte>, or ReadOnlyMemory<byte> properties get incomplete schemas — validators and tooling that rely on contentEncoding will not correctly identify the base64 encoding constraint.

Regression

No — this is a missing feature, not a regression.

Testing

All 387 JsonSchemaExporter tests pass, including the updated expectations for the three affected types.

Risk

Low. Changes are confined to three converters and the internal JsonSchema class. No behavior change to serialization/deserialization; only affects schema generation output.

Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.

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