chore(ci): add Python 3.15-dev to test-published matrix

[w-martin]

Fixes #11.

Summary

• Adds 3.15-dev to the test-published matrix with continue-on-error: true
• Adds allow-prereleases: true to actions/setup-python
• Provides early PyO3 ABI compatibility signal before Python 3.15 stable (~Oct 2026)

Test plan

• Workflow YAML is valid
• continue-on-error is conditional on 3.15-dev only
• Existing matrix legs (3.11–3.14) unaffected

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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