chore: bump Python target version from 3.11 to 3.12

[yankay]

What does this PR do?

1. Update target-version in pyproject.toml from py311 to py312.
2. Bumps the CI workflow Python version to 3.12.

Why is this change needed?

• Python 3.11 is no longer the current version
• Python 3.12 is the default version used by vLLM
• Ensures consistency with the broader llm-d ecosystem

How was this tested?

• Unit tests added/updated
• Integration/e2e tests added/updated
• Manual testing performed

Checklist

• Commits are signed off (git commit -s) per DCO
• Code follows project contributing guidelines
• Tests pass locally (make test)
• Linters pass (make lint)
• Documentation updated (if applicable)

Related Issues

[Copilot]

Pull request overview

Updates the repository’s Python tooling/CI configuration to target Python 3.12, aligning Ruff’s parsing/upgrade behavior and CI runtime with the newer Python baseline.

Changes:

• Bump Ruff target-version from py311 to py312 in pyproject.toml.
• Bump GitHub Actions actions/setup-python version from 3.11 to 3.12 for the Python lint job.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pyproject.toml	Updates Ruff’s target-version to Python 3.12.
.github/workflows/ci-pr-checks.yaml	Runs Python linting steps under Python 3.12 in CI.

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