chore: update llama-index package version requirements (#708)

## Summary

Update minimum version requirements for llama-index packages to match
the versions already installed as dependencies of litellm 1.80.0:
- **llama-index-llms-openai**: `0.6.5` → `0.6.12`
- **llama-index-llms-openai-like**: `0.3.3` → `0.5.3`

## Motivation

After upgrading litellm to 1.80.0 in #707, these llama-index packages
were automatically upgraded as transitive dependencies. This PR
explicitly updates the minimum version requirements in `pyproject.toml`
to:
- Document the actual versions being used
- Ensure compatibility with the latest OpenAI models
- Prevent accidental downgrades in future dependency resolutions

## Changes

- Updated `backend/pyproject.toml`:
  - `llama-index-llms-openai>=0.6.12` (was 0.6.5)
  - `llama-index-llms-openai-like>=0.5.3` (was 0.3.3)

## Testing

- ✅ Verified packages are already at these versions (installed as
litellm dependencies)
- ✅ No new packages need to be installed (`uv sync` completed with no
changes)
- ✅ Confirmed versions with `uv pip list`:
  - llama-index-llms-openai: 0.6.12
  - llama-index-llms-openai-like: 0.5.3

## Related PRs

- #707 - Initial litellm upgrade that brought in these versions as
dependencies

Author: Mini256

backend/pyproject.toml

@@ -50,8 +50,8 @@ dependencies = [
     "python-pptx>=1.0.2",
     "openpyxl>=3.1.5",
     "llama-index>=0.12.16",
-    "llama-index-llms-openai>=0.6.5",
-    "llama-index-llms-openai-like>=0.3.3",
+    "llama-index-llms-openai>=0.6.12",
+    "llama-index-llms-openai-like>=0.5.3",
     "llama-index-llms-bedrock-converse>=0.4.15",
     "llama-index-llms-ollama>=0.5.0",
     "llama-index-llms-google-genai>=0.1.6",