refactor(js/ai): moved generate api into ai package to enable use with middleware

[pavelgj]

GenkitAI APIs were refactored/moved unchanged into the ai package. This is necessary so that we can pass AI APIs into the middleware at runtime. Imagine usecases like context compression or request classification which require access to llms.

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Code Review

This pull request introduces the GenkitAI class to encapsulate core AI functionalities such as embedding, generation, and operation management. The Genkit class has been refactored to extend GenkitAI, inheriting these methods and reducing code duplication across the codebase. Feedback was provided regarding a misleading parameter name in the generateStream method to improve consistency with the generate method and the underlying implementation.