Fix MEAI001 leak from experimental RequiresConfirmation in source-generated AIContent contexts

[jeffhandley]

Tracks dotnet#7658

Problem

ToolApprovalRequestContent.RequiresConfirmation is [Experimental(MEAI001)], while ToolApprovalRequestContent is a stable, statically registered [JsonDerivedType] of the [JsonPolymorphic] AIContent and InputRequestContent hierarchies. This regression was introduced in 10.8.0 by Add ToolApprovalRequestContent.RequiresConfirmation (#7549).

As a result, System.Text.Json source generation emits the experimental member into the JSON metadata for any consumer context that reaches AIContent, even when the consumer never uses approval APIs:

[JsonSerializable(typeof(List<AIContent>))]
internal partial class JsonContext : JsonSerializerContext;

The build emits MEAI001 from source-generated code. Consumers are then forced into a project-wide suppression as the only practical remedy, but the suppression does not remove the member from their generated contract: their externally exposed JSON payloads still include the experimental requiresConfirmation field without an approval-feature opt-in.

Fix

ToolApprovalRequestContent is stable; only RequiresConfirmation is experimental. Keep the type in the static polymorphic contract, but gate its serialization at the member level using the established pattern used by UsageDetails token counts and HostedFileContent:

[Experimental(...)]
[JsonIgnore]
public bool RequiresConfirmation
{
    get => RequiresConfirmationCore;
    set => RequiresConfirmationCore = value;
}

[JsonInclude]
[JsonPropertyName("requiresConfirmation")]
internal bool RequiresConfirmationCore { get; set; } = true;

The internal member remains available to the package-owned default serialization path, so persisted conversations continue to round-trip requiresConfirmation. Consumer source-generated contracts omit the experimental field, and direct use of the public property continues to require MEAI001 opt-in.

The two Microsoft.Extensions.AI.Evaluation.Reporting projects had gained <NoWarn>$(NoWarn);MEAI001</NoWarn> alongside dotnet#7549 solely because of this leak. The fix removes both suppressions; their clean builds demonstrate that they were leak-driven. Abstractions and AI retain their suppressions because they legitimately use experimental APIs directly.

Regression coverage

Adds a no-suppression consumer-boundary guard in Microsoft.Extensions.AI.Stabilization.Tests:

• AIContentSerializationRegressionContext.cs source-generates List<AIContent>. Any experimental member leaking into the contract produces MEAI001 at compile time.
• AIContentSerializationRegressionTests.cs confirms stable List<AIContent> round-trips through that generated context.
• ToolApprovalRequestContent tests verify RequiresConfirmation still round-trips as both true and false through AIJsonUtilities.DefaultOptions and AIContent polymorphism.

Validation

• Microsoft.Extensions.AI.Stabilization.Tests: 90/90 pass across net472, net8.0, net9.0, and net10.0 without an MEAI001 suppression.
• Microsoft.Extensions.AI.Abstractions.Tests: 1634 pass; the intended persistence round-trip remains intact.
• Negative proof: restoring the original public serialized property produces MEAI001 in the generated AIContentSerializationRegressionContext.ToolApprovalRequestContent.g.cs across all four target frameworks.