ci: run the Python test suite — CONTRIBUTING.md asks for tests CI never executes (#100)

* ci: run the Python test suite - CONTRIBUTING.md asks for tests CI never executes

CONTRIBUTING.md tells contributors to put Python tool tests in tests/
and run the relevant suites, and tests/ now holds real ones
(test_salary_lookup.py, test_convert_salary_excel.py from #75) - but no
CI job executes them. A suite that never runs in CI can't gate a PR and
silently rots. New python-tests job: unittest discover over tests/,
stdlib only, no new dependencies. Future test files run without any
workflow change.

Also lands tests/test_security_guards.py, which missed #84's merge
window (pushed to the branch as #84 was being merged; the merge took
2a6cb8c, the tests were 260c37a). 13 unittest cases in the existing
tests/ style: each copies the guard script into a synthetic repo tree
and runs it as a subprocess - the same way CI invokes it - asserting
real exit codes and messages. Every forbidden state fails (Bash(*) and
Bash(curl:*) additions, each personal-data gitignore rule removed one
at a time, each forbidden lifecycle script, trustedDependencies,
invalid settings JSON, zero manifests); every non-event passes (dropped
shipped permission, extra ignore rules, benign scripts, hostile
manifest inside node_modules); and the real repo passes its own guards.

22 tests total, all passing locally via the exact command the job runs.

* test: use benign lifecycle-script values in fixtures - AV heuristics flag attack-shaped strings

Review found the curl-pipe-to-sh fixture value matches a real Defender
signature (Trojan:Script/Stealer.HAX!MTB): Windows quarantines the temp
package.json mid-test, making the suite flaky for any Windows
contributor who runs it - while proving nothing extra, since the guard
flags the script KEY and never inspects the value.

Fixture values are now 'echo test' (also in the node_modules-ignored
test, same class of string), with a comment on the key-only test
explaining why the value must stay benign so a future 'make the fixture
realistic' cleanup doesn't reintroduce the quarantine flake. Coverage
is unchanged: same keys, same assertions, 13 tests passing.

.github/workflows/ci.yml

@@ -54,6 +54,16 @@ jobs:
           python-version: "3.12"
       - run: python tools/security_guards.py
 
+  python-tests:
+    name: Python tool tests
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+      - uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5
+        with:
+          python-version: "3.12"
+      - run: python -m unittest discover -s tests -t . -v
+
   dependency-review:
     name: Dependency review (upstream PRs only)
     # Requires the repo's Dependency graph, which forks never inherit and