.NET: Surface downstream experimental flags and remove unnecessary suppressions (#3968)

* Surface downstream experimental flags and remove unecessary suppressions

* Fix format error

* Fix file encoding.

* Address PR comments.

dotnet/src/Shared/DiagnosticIds/DiagnosticsIds.cs

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+// Copyright (c) Microsoft. All rights reserved.
+
+namespace Microsoft.Shared.DiagnosticIds;
+
+/// <summary>
+///  Various diagnostic IDs reported by this repo.
+/// </summary>
+internal static class DiagnosticIds
+{
+    /// <summary>
+    ///  Experiments supported by this repo.
+    /// </summary>
+    internal static class Experiments
+    {
+        // This experiment ID is used for all experimental features in the Microsoft Agent Framework.
+        internal const string AgentsAIExperiments = "MAAI001";
+
+        // These diagnostic IDs are defined by the MEAI package for its experimental APIs.
+        // We use the same IDs so consumers do not need to suppress additional diagnostics
+        // when using the experimental MEAI APIs.
+        internal const string AIResponseContinuations = MEAIExperiments;
+        internal const string AIMcpServers = MEAIExperiments;
+        internal const string AIFunctionApprovals = MEAIExperiments;
+
+        // These diagnostic IDs are defined by the OpenAI package for its experimental APIs.
+        // We use the same IDs so consumers do not need to suppress additional diagnostics
+        // when using the experimental OpenAI APIs.
+        internal const string AIOpenAIResponses = "OPENAI001";
+        internal const string AIOpenAIAssistants = "OPENAI001";
+
+        private const string MEAIExperiments = "MEAI001";
+    }
+}

dotnet/src/Shared/DiagnosticIds/README.md

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+# Diagnostic IDs
+
+Defines various diagnostic IDs reported by this repo.
+
+To use this in your project, add the following to your `.csproj` file:
+
+```xml
+<PropertyGroup>
+  <InjectSharedDiagnosticIds>true</InjectSharedDiagnosticIds>
+</PropertyGroup>
+```