fix: `VertexAIGeminiGenerator` - remove support for tools and change output type

[anakin87]

Related Issues

• twin PR of #1177

We are removing support for tools in order to produce a List[str] output type, for consistency with other generators. Users interested in tool calling should use the corresponding Chat Generator.

Proposed Changes:

• remove support for tools
• change output type to List[str]

How did you test it?

CI

Notes for the reviewer

This is a breaking change and I will release a major version.

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.