fix(go-adk): openai embeddings generation (#2132)

Close #2131 

The main fix is to remove `HTTPClient` passed to embedding client so
that it creates its own and attaches auth header properly (one line
fix).

However, since we're here already, I'm also moving OpenAI, Azure, and
Ollama providers to use their respective SDK like Bedrock and Gemini
instead of doing direct HTTP calls to the endpoints. This ensures better
long-term stability and maintainability. Also improves some test
coverage + refactoring

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Signed-off-by: Jet Chiang <pokyuen.jetchiang-ext@solo.io>

Author: supreme-gg-gg