feat: use google/genai sdk for streaming gemini & vertex responses

[uinstinct]

Description

Use @google/genai sdk instead of fetching responses from the url. This correctly parses the response chunks without checking for json delimiterss

resolves CON-4266
closes #2651

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• [] I've read the contributing guide
• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created

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Summary by cubic

Switch Gemini and Vertex streaming to the @google/genai SDK for reliable chunk parsing and consistent streaming. Addresses Linear CON-4266 and fixes issues with JSON delimiter-based parsing.

• Refactors

  • Gemini adapter streams via the GoogleGenAI SDK and properly handles text, tool calls, thought signatures, and usage.
  • VertexAI adapter initializes GoogleGenAI (API key or Vertex project/region); streams Gemini via the SDK; Anthropic stays SSE; Mistral streams via the OpenAI adapter.
  • Core llms (Gemini, Vertex) route chat, embed, list, rerank, and streaming through the OpenAI adapter via useOpenAIAdapterFor.

• Dependencies

  • Added @google/genai ^1.30.0.

^{Written for commit fd793c4. Summary will update automatically on new commits.}

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✅ Addresses code review comments

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✅ Jira ticket created with business summary

🎫 View ticket: https://continue-playground.atlassian.net/browse/CRM-14

This ticket captures the business impact and user benefits of fixing the Gemini/Vertex streaming character loss issue.

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Fixed an incorrect documentation change in packages/openai-adapters/README.md.

The PR had incorrectly marked Bedrock and VertexAI adapters as unsupported (unchecked), but these adapters remain fully functional after the SDK migration. Restored the checkmarks to indicate continued support.

[cubic-dev-ai]

No issues found across 6 files

[RomneyDa]

@uinstinct sorry for delay on this one, could you rebase with main?

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