[codex] Stabilize generator CI and snapshot baselines

[HavenDV]

Summary

• emit shared options support per generated namespace in the source generator
• avoid emitting Newtonsoft property converter attributes for converters that are only generated for System.Text.Json
• refresh snapshot baselines and stale HTTP JSON assertions to match current generated output

Validation

• dotnet build src/libs/AutoSDK.SourceGenerators/AutoSDK.SourceGenerators.csproj /p:TreatWarningsAsErrors=true
• dotnet test src/tests/AutoSDK.SnapshotTests/AutoSDK.SnapshotTests.csproj --filter "FullyQualifiedName~ParallelGeneration"
• dotnet test src/tests/AutoSDK.SnapshotTests/AutoSDK.SnapshotTests.csproj --filter "FullyQualifiedName~SdkGenerator"
• dotnet test src/tests/AutoSDK.UnitTests/AutoSDK.UnitTests.csproj
• dotnet test src/tests/AutoSDK.IntegrationTests.Cli/AutoSDK.IntegrationTests.Cli.csproj --filter "FullyQualifiedNameGenerate_WithRequestAndClientOptions_EmitsOptionsSupportAndBuilds|FullyQualifiedNameGenerate_WithSystemNetHttpJsonFlag_UsesHttpJsonHelpers_AndBuilds|FullyQualifiedNameGenerate_WithOpenApi32QueryStringCookieAndAdditionalOperations_BuildsAndEmitsSupport|FullyQualifiedNameGenerate_WithOpenApi32SequentialMediaTypesAndItemSchema_BuildsAndEmitsStreamingSupport|FullyQualifiedName~Generate_WithServerVariableDefaults_UsesResolvedBaseUrl"