ci: expand Python version matrix to 3.11, 3.12, 3.13

[shaypal5]

Summary

• Adds a strategy.matrix to the validate job with python-version: ["3.11", "3.12", "3.13"]
• Sets fail-fast: false so all three versions always run to completion
• Replaces the hardcoded "3.12" in setup-python with ${{ matrix.python-version }}

Test plan

• All three Python versions run validate_indexes.py, generate_release_artifacts.py --check, and pytest
• No regressions on 3.12
• If 3.13 surfaces noise from jsonschema or frictionless, drop it, leave a comment in the YAML, and close noting the drop

Closes #2

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