[Vertex AI] Enable AccessLevelOnImport and InternalImportsByDefault

[andrewheard]

WIP - Blocked on #14627 and #14638

Enabled the experimental (in Swift 5.x) feature AccessLevelOnImport and the upcoming opt-in feature InternalImportsByDefault for the Vertex AI SDK. AccessLevelOnImport will be the default in Swift 6.x and can be removed once #14627 and #14638 have been completed.

This PR starts resolving #13031 by removing uses of @_implementationOnly in the FirebaseVertexAI target.

#no-changelog

[gemini-code-assist]

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point in your pull request via creating an issue comment (i.e. comment on the pull request page) using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands.

Feature	Command	Description
Code Review	/gemini review	Performs a code review for the current pull request in its current state.
Pull Request Summary	/gemini summary	Provides a summary of the current pull request in its current state.
Comment	@gemini-code-assist	Responds in comments when explicitly tagged, both in issue comments and review comments.
Help	/gemini help	Displays a list of available commands.

Customization

To customize Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist is currently in preview and may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counter productive. You can react with 👍 and 👎 on @gemini-code-assist comments to provide feedback.

[paulb777]

Is this PR still relevant?

[ncooke3]

Yes, but the Package.swift changes can wait for Firebase 12. And internal could explicitly be added to imports for now.

[andrewheard]

Yes, but the Package.swift changes can wait for Firebase 12. And internal could explicitly be added to imports for now.

@ncooke3 @paulb777 If I can do it quickly, I'd like to see if I can continue to use InternalImportsByDefault (removed AccessLevelOnImport though). I'd be more comfortable that I haven't missed something and unintentionally made it public.