Add Cosmos DB user agent suffixes

[prsasattms]

Summary

• add OmniVec-MetadataCosmos/1.0 for metadata and lease Cosmos clients
• add OmniVec-DataCosmos/1.0 for data Cosmos clients
• keep the code diff additive-only: 44 insertions, 0 deletions

Validation

• python -m compileall -q api
• dotnet build connectors\ingestion\dotnet\OmniVec.ChangeFeed.csproj -v:minimal
• dotnet build connectors\worker\dotnet\OmniVec.Worker.csproj -v:minimal