chore: upgrade litellm to 1.77.5+ to support latest OpenAI models (#707)

## Summary

- Upgrade litellm dependency to version 1.77.5+ to support newer OpenAI
models
- Backend: litellm `1.67.4` → `1.80.0`
- Core: litellm `1.61.11` → `1.80.13`

## Motivation

The current litellm version doesn't recognize newer OpenAI models like
GPT-5.2 series, causing "Unknown model" errors when using OpenAI-like
providers. This update adds support for:

- GPT-5.2 series models (gpt-5.2-instant, gpt-5.2-thinking, gpt-5.2-pro)
- Other recent OpenAI model releases
- Improved compatibility with latest model APIs

## Changes

- Updated `backend/pyproject.toml`: `litellm>=1.77.5`
- Updated `core/pyproject.toml`: `litellm>=1.77.5`
- Regenerated lock files with `uv sync`

## Testing

- ✅ Verified litellm imports successfully
- ✅ Verified OpenAILike integration works
- ✅ Confirmed versions: backend (1.80.0), core (1.80.13)

## References

- LiteLLM v1.77.5 release notes:
https://docs.litellm.ai/release_notes/v1-77-5
- OpenAI GPT-5.2 announcement

Author: Mini256

backend/pyproject.toml

@@ -68,7 +68,7 @@ dependencies = [
     "llama-index-llms-vertex>=0.4.2",
     "socksio>=1.0.0",
     "dspy>=2.6.21",
-    "litellm>=1.67.4",
+    "litellm>=1.77.5",
 ]