Fix trim/AOT safety for custom event argument deserialization in Blazor

[NanthiniMahalingam]

Bug description

• When a Blazor component uses a custom event (registered via [EventHandler]) whose event-args type is deserialized from JSON, the members of that type could be trimmed away in trimmed/AOT-published apps. The deserialization path in WebEventData relied on JsonSerializer.Deserialize(string, Type, options), which is annotated RequiresUnreferencedCode.
• That warning was being hidden with an UnconditionalSuppressMessage rather than actually being made trim-safe, so custom event args could fail to deserialize at runtime after trimming.

Root cause

• The custom event-args Type flows from [EventHandler(..., Type eventArgsType)] → Renderer.GetEventArgsType(...) → JSON deserialization, but the Type was not annotated with [DynamicallyAccessedMembers]. The trimmer therefore had no signal to preserve the JSON-required members.
• WebEventData.ParseEventArgsJson used the reflection-based JsonSerializer.Deserialize overload (RequiresUnreferencedCode) and merely suppressed the IL2026 warning, masking the real trimming hole instead of closing it.

Description of code changes

• Annotated the eventArgsType constructor parameters and the EventArgsType property on EventHandlerAttribute.cs with DynamicallyAccessedMembers(LinkerFlags.JsonSerialized), so the members needed for JSON (de)serialization are preserved through trimming.
• In WebEventData.cs, switched to the trim-safe API: resolve JsonTypeInfo via jsonSerializerOptions.GetTypeInfo(eventArgsType) and deserialize with that, removing the now-unnecessary UnconditionalSuppressMessage suppression.

Fixes #45851

Output

Before changes

beforefix-45851.mp4

After changes

afterfix-45851.mp4

[dotnet-policy-service]

Thanks for your PR, @NanthiniMahalingam. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.