Add simplified [JsonSerializable] source generation for POCOs

[Copilot]

Description

Today, System.Text.Json source generation requires creating a separate JsonSerializerContext-derived class and annotating it with [JsonSerializable(typeof(T))] for every type — significantly more ceremony than the reflection API. This PR allows placing [JsonSerializable] directly on a partial class or struct to auto-generate a backing context and a static JsonTypeInfo property.

Before:

[JsonSerializable(typeof(WeatherForecast))]
internal partial class MyContext : JsonSerializerContext { }

string json = JsonSerializer.Serialize(forecast, MyContext.Default.WeatherForecast);

After:

[JsonSerializable]
public partial class WeatherForecast
{
    public string City { get; set; }
    public int Temperature { get; set; }
}

string json = JsonSerializer.Serialize(forecast, WeatherForecast.JsonTypeInfo);

Attribute changes

• Added parameterless constructor overload to JsonSerializableAttribute
• Expanded AttributeTargets to include Struct
• Updated ref assembly
• Added API compat suppressions (CP0015) for AttributeUsageAttribute change on JsonSerializableAttribute in both ApiCompatBaseline.NetCoreAppLatestStable.xml and per-package CompatibilitySuppressions.xml

Source generator pipeline

• Added second incremental pipeline in JsonSourceGenerator.Roslyn4.0.cs and equivalent logic in Roslyn3.11.cs that detects parameterless [JsonSerializable] on non-context types
• Parser produces a (PocoTypeGenerationSpec, ContextGenerationSpec) tuple using the existing type graph traversal, so the full emitter infrastructure is reused
• Added TypeRef(string, string) constructor for synthetic context types that don't exist in the compilation

Emitter output (per annotated type)

• __JsonContext_<TypeName>.Base.g.cs — base class declaration for the synthetic context
• __JsonContext_<TypeName>.g.cs + per-type files — full context implementation via existing Emit() path
• <TypeName>.JsonSerializable.g.cs — partial type extension with public static JsonTypeInfo<T> JsonTypeInfo property

Diagnostics

• SYSLIB1227 — type must be declared partial
• SYSLIB1228 — member name collision on JsonTypeInfo (use TypeInfoPropertyName to resolve)
• Existing SYSLIB1224 now only fires for [JsonSerializable(typeof(T))] on non-context types, not for the parameterless form

Tests

• 7 new unit tests covering class, struct, non-partial diagnostic, name collision diagnostic, custom TypeInfoPropertyName, coexistence with explicit context, and global namespace
• All 218 existing unit tests pass

Not included (follow-up)

• Roslyn interceptor to rewrite JsonSerializer.Serialize(val) → JsonSerializer.Serialize(val, T.JsonTypeInfo) (plan item 4 — requires significant interceptor infrastructure)

[agocke]

@copilot why are we disabling diagnostic validation?

[Copilot]

These tests intentionally produce Warning-level diagnostics (SYSLIB1227 for non-partial type, SYSLIB1228 for member name collision). The default validation in RunJsonSourceGenerator asserts AssertMaxSeverity(DiagnosticSeverity.Info), which would fail for these cases. This is the same pattern used by other diagnostic tests in this file (lines 211, 231, 987).

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-text-json
See info in area-owners.md if you want to be subscribed.

[agocke]

@copilot

src/libraries/apicompat/ApiCompat.proj(92,5): error CP0015: (NETCORE_ENGINEERING_TELEMETRY=Build) Cannot change arguments of attribute 'System.AttributeUsageAttribute' on 'System.Text.Json.Serialization.JsonSerializableAttribute'.

I think we need to update the API compat files to allow the new attribute usage

[Copilot]

@copilot

src/libraries/apicompat/ApiCompat.proj(92,5): error CP0015: (NETCORE_ENGINEERING_TELEMETRY=Build) Cannot change arguments of attribute 'System.AttributeUsageAttribute' on 'System.Text.J...

Added CP0015 suppressions for JsonSerializableAttribute's AttributeUsageAttribute change in both ApiCompatBaseline.NetCoreAppLatestStable.xml and the per-package CompatibilitySuppressions.xml (for all three TFMs: net10.0, net462, netstandard2.0). See bf56845.