[Dependencies] Update System.ClientModel to version 1.8.1

[joseharriaga]

Update System.ClientModel to version 1.8.1 as follows:

1. In src/OpenAI.csproj, change:

<PackageReference Include="System.ClientModel" Version="1.8.0" />

to

<PackageReference Include="System.ClientModel" Version="1.8.1" />

2. In scripts/Export-Api.ps1, update the assembly reference resolution logic for System.ClientModel:

• Change any references to previous versions (such as $systemClientModelPath = Join-Path $nugetPackagesPath "system.clientmodel\1.7.0") to 1.8.1 in path resolution and related commands if present.

Files to modify:

• src/OpenAI.csproj
• scripts/Export-Api.ps1