ci: add python 3.15 and run lint and tests from one workflow

[WilliamBergamin]

Summary

This pull request adds Python 3.15 to CI and consolidates linting and testing into a single .github/workflows/ci.yml workflow named CI, matching the other Bolt for Python samples.

• lint job: ruff check . and ruff format --check ., on Python 3.15.
• test job: pytest across Python 3.11 to 3.15.
• Removes tests.yml.
• Lint moves out of the matrix into its own job, so it runs once. The required test (3.x) check names are unchanged.

Testing

• Confirm the CI / lint and CI / test checks appear and pass in this PR.
• Ran ruff check ., ruff format --check ., and pytest . -v on a local Python 3.15.0 venv: passed (13 tests).

🤖 Generated with Claude Code

[zimeg]

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