ci: Require 100% code coverage

Now that every line and branch in injector is exercised, raise the
--cov-fail-under gate from 90 to 100 so coverage regressions fail PRs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: KrzysztofMadejski

pytest.ini

@@ -1,3 +1,3 @@
 [pytest]
-addopts = -v --tb=native --doctest-glob=*.md --doctest-modules --cov-report term --cov-report html --cov-report xml --cov=injector --cov-branch --cov-fail-under=90
+addopts = -v --tb=native --doctest-glob=*.md --doctest-modules --cov-report term --cov-report html --cov-report xml --cov=injector --cov-branch --cov-fail-under=100
 norecursedirs = __pycache__ *venv* .git build