feat: add auto-generate models workflow

[nearlyforget]

Description

This PR adds an automated workflow to handle specification releases and updates the generation script for better CI compatibility.

Changes:

• Added .github/workflows/auto-generate.yml to listen for spec-release events.
• Updating the generation script to support the SCHEMA_DIR environment variable makes the script portable. It allows the SDK to be built in diverse environments (like CI/CD pipelines or different
  local folder structures) without hardcoding a relative path that might not always be accurate.
• Automated version bumping in pyproject.toml and PR creation.

Motivation:
This allows the Python SDK to stay up-to-date with the UCP specification automatically, reducing manual maintenance and ensuring type safety with the latest schemas.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code