.Net: Bug: labels are not correctly implemented in vertex ai

[schmitch]

Describe the bug
currently here: #12300 (#12257) labels were created with a string field, unfortunatly in the vertex ai api docs: https://cloud.google.com/vertex-ai/docs/reference/rest/v1/projects.locations.endpoints/generateContent

they are a key value pair. @VladyslavLishchyna also asked the same question inside the merge request.

To Reproduce

create a small example that will set the labels and it will not work, since google only wants a key value pair.

Expected behavior
labels should be a dictionary<string, string>

Screenshots
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Platform

• Language: c#
• Source: latest preview
• AI model: Vertex AI Gemini Flash 2.5

Additional context
Add any other context about the problem here.

[shethaadit]

Hi @rogerbarreto / @markwallace-microsoft, I have fixed this. Could you please help reviewing PR?