Add bug-finding lint to the checks

Run ruff with pyflakes, syntax-error and bugbear rules only (no style
rules) over the integration, tests and scripts as part of check.sh and
CI. The codebase passes as is. Target Python 3.9 because the offline
tests are also run on older interpreters, which also leaves out the
zip(strict=) rule that needs 3.10. ruff 0.14.0 pinned in both test
requirement files.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ruff.toml

@@ -0,0 +1,6 @@
+# Bug-finding lint only (pyflakes, syntax errors, bugbear); no style rules.
+# py39: the offline tests also run on older Pythons, so e.g. zip(strict=) is not available.
+target-version = "py39"
+
+[lint]
+select = ["F", "E9", "B"]