Python: add non-blocking Python 3.15 CI - #31

Motivation & Context

Python 3.15 is in release-candidate status and needs early compatibility coverage without making branch protection depend on prerelease ecosystem readiness. Running it in CI now surfaces interpreter and dependency issues before support becomes blocking.

Description & Review Guide

• What are the major changes? Adds Python 3.15 to the core Python test matrix and applies continue-on-error only to the 3.15 matrix jobs.
• What is the impact of these changes? Python 3.15 tests run on Ubuntu and Windows without blocking pull requests, while Python 3.10 through 3.14 remain required. Lab CI is unchanged.
• What do you want reviewers to focus on? Confirm that the conditional allowed-failure behavior is scoped only to Python 3.15 and preserves the existing blocking behavior for supported versions.

Related Issue

N/A — intentionally created without a linked issue.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.