chore(deps-dev): bump llama-index in the llama group

Bumps the llama group with 1 update: [llama-index](https://github.com/run-llama/llama_index).


Updates `llama-index` from 0.12.7 to 0.12.8
- [Release notes](https://github.com/run-llama/llama_index/releases)
- [Changelog](https://github.com/run-llama/llama_index/blob/main/CHANGELOG.md)
- [Commits](run-llama/llama_index@v0.12.7...v0.12.8)

---
updated-dependencies:
- dependency-name: llama-index
  dependency-type: direct:development
  update-type: version-update:semver-patch
  dependency-group: llama
...

Signed-off-by: dependabot[bot] <[email protected]>