Switch C# Responses client to official OpenAI package

Replaces the hand-rolled Responses DTO/SSE/serialization layer with the official `OpenAI` 2.10.0 NuGet package's `OpenAI.Responses.ResponsesClient` while keeping `Microsoft.AI.Foundry.Local.OpenAIResponsesClient` as the public Foundry Local-shaped wrapper.

- Public surface unchanged: `OpenAIResponsesClient` ctor, `Settings`, `CreateAsync` / `CreateStreamingAsync` / `GetAsync` / `DeleteAsync` / `CancelAsync` / `GetInputItemsAsync` / `ListAsync`

- Endpoints come from `OpenAI.Responses.ResponsesClient` configured with the Foundry Local web service URL via `OpenAIClientOptions.Endpoint`; only `ListAsync` keeps a small `HttpClient` shim for Foundry Local's list-responses extension (the official client doesn't expose that yet)

- `ResponsesClientSettings` keeps the same Foundry Local-shaped knobs but applies them onto official `CreateResponseOptions` (`MaxOutputTokenCount`, `StoredOutputEnabled`, `ParallelToolCallsEnabled`, etc.)

- New `ResponseContentPartHelpers` produces official `ResponseContentPart` instances for vision: file (auto-detect MIME, throws on missing/unknown extension), bytes (data URI), and URL

- Vision now uses official OpenAI shape (`input_image` + `image_url` data URI) rather than Foundry Local's `image_data` + `media_type` extension

- Deleted the hand-rolled `ResponsesTypes.cs` polymorphic DTO surface and the source-gen `ResponsesSerializationContext`

Bug fixes:

- Avoid `op_Implicit(null)` on `ResponseImageDetailLevel` and `ResponseItemCollectionOrder` value-type structs when callers pass null/empty options

- Map `ClientResultException` -> `FoundryLocalException` consistently across CRUD methods

Tests:

- Unit tests rewritten for the official surface; cover settings defaults, input validation, image helper factories, MIME detection, and `ListResponsesResult.FromJson` parsing

- Integration tests updated to use `CreateResponseOptions` / `ResponseItem` / `StreamingResponseUpdate` types

- `dotnet build` clean; all Responses tests pass (51/61 overall, the 10 failures are pre-existing `EmbeddingClientTests` infra)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: MaanavD

sdk/cs/src/Microsoft.AI.Foundry.Local.csproj

@@ -72,8 +72,7 @@
     <!-- This target runs automatically after package assets are resolved and prints the exact version of the Core package that was selected. -->
     <Target Name="PrintResolvedVersions" AfterTargets="ResolvePackageAssets">
         <Message Importance="High" Text="Resolved Dependencies:" />
-        <Message Importance="High" Text="  %(PackageDependencies.Identity) : %(PackageDependencies.Version)" 
-                 Condition="$([System.String]::Copy('%(PackageDependencies.Identity)').StartsWith('Microsoft.AI.Foundry.Local.Core'))" />
+        <Message Importance="High" Text="  %(PackageDependencies.Identity) : %(PackageDependencies.Version)" Condition="$([System.String]::Copy('%(PackageDependencies.Identity)').StartsWith('Microsoft.AI.Foundry.Local.Core'))" />
     </Target>
 
     <ItemGroup>
@@ -121,14 +120,13 @@
       <NoWarn>$(NoWarn);NU1604</NoWarn>
     </PropertyGroup>
     <ItemGroup>
-      <PackageReference Condition="'$(UseWinML)' == 'true'"
-                        Include="Microsoft.AI.Foundry.Local.Core.WinML" Version="$(FoundryLocalCoreWinMLVersion)" />
-      <PackageReference Condition="'$(UseWinML)' != 'true'"
-                        Include="Microsoft.AI.Foundry.Local.Core" Version="$(FoundryLocalCoreVersion)" />
+      <PackageReference Condition="'$(UseWinML)' == 'true'" Include="Microsoft.AI.Foundry.Local.Core.WinML" Version="$(FoundryLocalCoreWinMLVersion)" />
+      <PackageReference Condition="'$(UseWinML)' != 'true'" Include="Microsoft.AI.Foundry.Local.Core" Version="$(FoundryLocalCoreVersion)" />
 
       <PackageReference Include="Betalgo.Ranul.OpenAI" Version="9.1.0" />
       <PackageReference Include="Microsoft.Extensions.Logging" Version="9.0.9" />
       <!-- specify PrivateAssets to exclude from nuget dependencies -->
-      <PackageReference Include="IDisposableAnalyzers" Version="4.0.8" PrivateAssets="all"/>
+      <PackageReference Include="IDisposableAnalyzers" Version="4.0.8" PrivateAssets="all" />
+      <PackageReference Include="OpenAI" Version="2.10.0" />
     </ItemGroup>
 </Project>