deps: bump chainlit to 2.12.0 in chat-chainlit-app example

[katsugtgz]

Updates chainlit to address GHSA-w3fx-mc44-mf6j and GHSA-hvfh-5mj3-5f3j in the chat-chainlit-app example.

Evidence:

• examples/chat-chainlit-app/requirements.txt referenced chainlit@2.9.4
• osv-scanner reported both advisories for chainlit@2.9.4
• updated version: 2.12.0

Validation:

• osv-scanner no longer reports chainlit advisories after the update
• pip install -r requirements.txt (with the bumped pins) succeeds in a clean venv on Python 3.11

Note: chainlit 2.12.0 requires pydantic >=2.11.0, so the pydantic pin moves from 2.10.1 to 2.11.0. Both are within pydantic 2.x.

Scope: dependency update only.