Support manual tag releases & add sanity checks

[C-Achard]

Publish workflow tweaks

• Add workflow_dispatch input (release_tag) and validate its format so the workflow can be run manually for a specific tag.
• Update checkout steps to use the provided tag for manual runs, and relax job guards to allow workflow_dispatch to trigger release/build/publish jobs.
• Add per-run validation steps that ensure a tag-like value is provided.
• Bump publish job Python to 3.12
• Add an explicit pre-upload existence check for the TWINE_API_KEY secret (safe), and remove the twine --verbose flag.
• Update the workflow usage comment to mention manual re-runs.

[Copilot]

Pull request overview

Updates the Python package build/release GitHub Actions workflow to support manually-triggered (workflow_dispatch) builds/publishes for a specified release tag, with added sanity checks and a more modern publish environment.

Changes:

• Add workflow_dispatch input release_tag, validate it, and use it to checkout the intended tag for manual runs.
• Relax build_release/publish job guards to allow manual runs in addition to tag pushes.
• Update publishing to Python 3.12, add a pre-upload credential presence check, and remove twine --verbose.

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[deruyter92]

Great work. Good that you added the manual dispatch as well