Backport: feat(provider/google): support Google search grounding when using generateImage with Gemini

[github-actions]

This is an automated backport of #15522 to the release-v6.0 branch. FYI @felixarntz
This backport has conflicts that need to be resolved manually. Conflicts resolved.

git cherry-pick output

Auto-merging content/providers/01-ai-sdk-providers/15-google.mdx
Auto-merging packages/google/src/google-generative-ai-image-model.test.ts
Auto-merging packages/google/src/google-generative-ai-image-model.ts
CONFLICT (modify/delete): packages/google/src/google-image-model-options.ts deleted in HEAD and modified in b71c0d7f4 (feat(provider/google): support Google search grounding when using `generateImage` with Gemini (#15522)).  Version b71c0d7f4 (feat(provider/google): support Google search grounding when using `generateImage` with Gemini (#15522)) of packages/google/src/google-image-model-options.ts left in tree.
Auto-merging packages/google/src/tool/google-search.ts
error: could not apply b71c0d7f4... feat(provider/google): support Google search grounding when using `generateImage` with Gemini (#15522)
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config set advice.mergeConflict false"

[github-actions]

🚀 Published in:

Package	Version
@ai-sdk/google	3.0.79 github npm
@ai-sdk/google-vertex	4.0.135 github npm