.Net: Azure.AI.OpenAI SDK V2 migration pre-release steps/checks

[SergeyMenshykh]

• Review all modified projects where files were excluded from the compilation process. Ensure that they no longer have excluded files/classes. Excluded files are listed as Compile items with the Remove attribute.

    <ItemGroup>
      <Compile Remove="Agents\ChatCompletion_FunctionTermination.cs" />
      <Compile Remove="Agents\ChatCompletion_Streaming.cs" />
    </ItemGroup>

• Remove the Phase * comments like this one: https://github.com/microsoft/semantic-kernel/blob/d436430e37f7401bb86bec1f74cee3aed01d7cde/dotnet/src/Connectors/Connectors.OpenAIV2/Core/ClientCore.cs#L3C1-L16C3 from all the files.
• Look at the PRs for the Connectors.OpenAI project functionality merged into the main branch from the day the migration started and copy those changes to the Connectors.OpenAIV2 project. Same for relevant unit and integration tests.
• Decide on the best way to merge Connectors.OpenAI and Connectors.OpenAIV2, or replace one with the other. If possible, prefer to keep the existing Connectors.OpenAI project and move new/changed classes from Connectors.OpenAIV2 into it. This will allow us to preserve files change history, which might be useful.
• Update to the latest OpenAI and Azure.AI.OpenAI SDK NuGet packages.
• Remove the VersionOverride="2.0.0-beta.2" from the Connectors.AzureOpenAI project. Ensure the old Azure.AI.OpenAI SDK NuGet package is not referenced or used anywhere.
• Ensure the assembly name - <AssemblyName>Microsoft.SemanticKernel.Connectors.OpenAIV2</AssemblyName> for the Connectors.OpenAIV2 project, if it's decided to keep it, does not have the V2 suffix.
• Enable EnablePackageValidation for Connectors.OpenAI project back.