Run pull-request AWS tests on the newest Python version only

[laughingman7743]

Use case

After #837, a ready pull request still runs every selected suite on all supported Python versions (3.10–3.14): 5 jobs for the PyAthena suite, plus 10 when the SQLAlchemy tests are selected. Most changes do not depend on the Python version, and version-specific regressions are rare.

Proposed change

• On pull requests, run the AWS suites only on the newest supported Python version.
• Run the full Python matrix on the weekly schedule and on manual dispatch. Before a release, the full matrix is enforced by Require a full test run on the release commit before publishing #851, which should land first.
• Add an input to .github/workflows/test-suite.yaml for the Python versions, and let test.yaml choose the versions by event.
• Keep a way to request the full matrix on a pull request, for example gh workflow run test.yaml --ref <branch> (dispatch already runs every suite).
• Update docs/testing.md.

Trade-off: a regression that only affects an older Python version is found by the weekly run or the pre-release run instead of on the pull request.

Validation plan (if implementing)

• On a ready pull request, only the newest Python runs.
• A dispatch run on a branch runs all versions.
• The weekly run continues to run all versions.