Azure OpenAI: Make startGenerateImage LRO internal for clients

[trrwilson]

This change introduces a client.tsp layer for Azure OpenAI's TypeSpec code generation basis and adds an @internal decorator to make its new image generation long-running operation separate from the public surface.

Image generation is not a long-running operation on non-Azure OpenAI and there's limited utility in having a divergent representation that exposes operation polling; this is principally an implementation detail of how Azure achieves the same result.

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• ️✔️Succeeded [Logs]Release - Generate from 582af8c. SDK Automation 14.0.0

  command	pwsh ./eng/scripts/Automation-Sdk-Init.ps1 ../azure-sdk-for-net_tmp/initInput.json ../azure-sdk-for-net_tmp/initOutput.json
  command	pwsh ./eng/scripts/Invoke-GenerateAndBuildV2.ps1 ../azure-sdk-for-net_tmp/generateInput.json ../azure-sdk-for-net_tmp/generateOutput.json

• ️✔️Azure.AI.OpenAI [View full logs]  [Release SDK Changes]
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Language	Package Name	ApiView Link
.Net	Azure.AI.OpenAI	https://apiview.dev/Assemblies/Review/2e8a1dd449f64ff6b53bb7d7bdc1632b