Generator: renamed/missing connector namespaces cause CS0234/CS0246 after regeneration

[daviburg]

Problem

When regenerating all connectors, 3 connectors produce CS0234/CS0246 errors because:

1. ConnectorServiceCollectionExtensions.cs (hand-authored) references namespaces PdfCo, SigningHub, and Yammer that no longer exist after regeneration — these connectors were renamed in the connector ecosystem.

2. CognitiveservicescustomvisionExtensions.cs references a PredictImageRequest type that is not generated (swagger schema issue).

3. ConversionserviceExtensions.cs references an HtmlToTextRequest type that is not generated (swagger schema issue).

Proposed Fix

1. Update ConnectorServiceCollectionExtensions.cs to use the new connector namespace names, or remove the entries for decommissioned connectors.
2. File swagger corrections for CognitiveServicesCustomVision and ConversionService connectors.

Context

Discovered during SDK regeneration for Azure SDK guideline compliance (#156, #157, #161, #175).