fix(go-adk): openai embeddings generation

[supreme-gg-gg]

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

[Copilot]

Pull request overview

Fixes Go ADK memory embedding failures (401 invalid_issuer) by ensuring embedding providers don’t reuse the Kagent-authenticated HTTP client, and refactors embedding generation to use vendor SDKs (OpenAI, Azure OpenAI, Ollama) for improved stability.

Changes:

• Stop passing a Kagent-authenticated HTTPClient into the embedding client so OpenAI auth headers are sourced correctly from OPENAI_API_KEY.
• Refactor embedding providers to use the OpenAI, Azure OpenAI (via OpenAI SDK), and Ollama SDK clients instead of manual HTTP calls.
• Add embedding package unit tests validating OpenAI API key usage and shared embedding post-processing behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
go/adk/pkg/memory/kagent_service.go	Stops injecting the Kagent HTTP client into embeddings to avoid incorrect auth propagation.
go/adk/pkg/memory/kagent_service_test.go	Updates mock embedding client creation to match the new embedding client configuration.
go/adk/pkg/embedding/embedding.go	Migrates OpenAI/Azure/Ollama embedding calls to SDKs and centralizes embedding post-processing.
go/adk/pkg/embedding/embedding_test.go	Adds targeted tests for OpenAI auth header usage and embedding normalization/validation helpers.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.