Add support for Gemini 2.5 flash stable (#6692)

As Gemini 2.5 Flash was released as stable the model infos should be
changed accordingly.

See https://ai.google.dev/gemini-api/docs/models?hl=de#gemini-2.5-flash

## Related issue number

No issue

## Checks

- [x] I've included any doc changes needed for
<https://microsoft.github.io/autogen/>. See
<https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md> to
build and test documentation locally.
- [x] I've added tests (if relevant) corresponding to the changes
introduced in this PR.
- [x] I've made sure all auto checks have passed.

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Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>