chore: support python 3.15

[WilliamBergamin]

Summary

This pull request adds Python 3.15 to the supported versions now that Python 3.15.0 is out.

• Adds 3.15 to the unit test matrix in ci-build.yml, keeping every existing entry.
• Sets LATEST_SUPPORTED_PY to 3.15 in ci-build.yml and in scripts/install_all_and_run_tests.sh, so lint, typecheck, and code coverage run on 3.15.
• Adds the Programming Language :: Python :: 3.15 classifier to pyproject.toml.
• Updates the 3.7--3.14 matrix mentions in AGENTS.md to 3.7--3.15.

Testing

• Confirm the new 3.15 unit test job appears and passes in this PR's checks.
• Confirm the lint, typecheck, and code coverage jobs now run on Python 3.15 and pass.
• Ran ./scripts/install_all_and_run_tests.sh on a local Python 3.15.0 venv: passed (format, lint, mypy clean; 953 passed).

Category

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.57%. Comparing base (abb6950) to head (ef0d23b).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1600   +/-   ##
=======================================
  Coverage   91.57%   91.57%           
=======================================
  Files         228      228           
  Lines        7285     7285           
=======================================
  Hits         6671     6671           
  Misses        614      614           

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