.NET: Surface downstream experimental flags and remove unnecessary suppressions

[westey (westey-m)]

Motivation and Context

#2542

Description

• Surface downstream experimental flags in core packages
• Remove unnecessary suppressions

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR surfaces downstream experimental flags from Microsoft.Extensions.AI (MEAI001) and OpenAI SDK (OPENAI001) by adding Experimental attributes to relevant APIs in the Agent Framework, and removes unnecessary diagnostic suppressions that are no longer needed.

Changes:

• Introduces a shared DiagnosticIds class that defines experimental diagnostic IDs matching downstream packages for consistency
• Adds Experimental attributes to OpenAI Response/Assistant extensions and AzureAI extensions
• Adds Experimental attributes to ContinuationToken-related properties in Abstractions
• Removes MEAI001/OPENAI001 NoWarn suppressions from projects where experimental attributes now properly surface the warnings

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
dotnet/src/Shared/DiagnosticIds/DiagnosticsIds.cs	New shared class defining experimental diagnostic IDs that match downstream packages
dotnet/src/Shared/DiagnosticIds/README.md	Documentation for using the DiagnosticIds shared file
dotnet/eng/MSBuild/Shared.props	MSBuild integration to inject DiagnosticIds into projects
dotnet/src/Microsoft.Agents.AI.OpenAI/Microsoft.Agents.AI.OpenAI.csproj	Enables DiagnosticIds injection, removes OPENAI001 suppression
dotnet/src/Microsoft.Agents.AI.OpenAI/Extensions/*.cs	Adds Experimental attributes to OpenAI Response and Assistant extension classes
dotnet/src/Microsoft.Agents.AI.AzureAI/Microsoft.Agents.AI.AzureAI.csproj	Enables DiagnosticIds injection
dotnet/src/Microsoft.Agents.AI.AzureAI/AzureAIProjectChatClientExtensions.cs	Adds Experimental attribute, removes pragma suppressions
dotnet/src/Microsoft.Agents.AI.AzureAI/AzureAIProjectChatClient.cs	Adds Experimental attribute, removes pragma suppressions
dotnet/src/Microsoft.Agents.AI.Abstractions/Microsoft.Agents.AI.Abstractions.csproj	Enables DiagnosticIds injection
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentRunOptions.cs	Adds Experimental attribute to ContinuationToken property
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentResponse.cs	Adds Experimental attribute to ContinuationToken property
dotnet/src/Microsoft.Agents.AI.DurableTask/Microsoft.Agents.AI.DurableTask.csproj	Removes MEAI001 suppression
dotnet/src/Microsoft.Agents.AI.CosmosNoSql/Microsoft.Agents.AI.CosmosNoSql.csproj	Removes MEAI001 suppression
dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/Microsoft.Agents.AI.Hosting.OpenAI.csproj	Removes OPENAI001 suppression
dotnet/tests/Microsoft.Agents.AI.CosmosNoSql.UnitTests/Microsoft.Agents.AI.CosmosNoSql.UnitTests.csproj	Removes MEAI001 suppression
dotnet/samples/HostedAgents/AgentWithHostedMCP/AgentWithHostedMCP.csproj	Removes MEAI001/OPENAI001 suppressions