chore(deps): bump gradio from 6.9.0 to 6.15.0 in /envs/echo_env

Bumps [gradio](https://github.com/gradio-app/gradio) from 6.9.0 to 6.15.0.
- [Release notes](https://github.com/gradio-app/gradio/releases)
- [Changelog](https://github.com/gradio-app/gradio/blob/main/CHANGELOG.md)
- [Commits](https://github.com/gradio-app/gradio/compare/gradio@6.9.0...gradio@6.15.0)

---
updated-dependencies:
- dependency-name: gradio
  dependency-version: 6.15.0
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]