Fix ruff linter errors in test_container_shell

SLF001 (private member accessed) is expected in tests that exercise
module internals directly. Suppress it via per-file-ignores for tests/*.

PLW0603 (global statement used for assignment) is the correct pattern
for the module-level container ID state. Add to the global ignore list
alongside the existing PLW0602 exemption.

Author: anticomputer

pyproject.toml

@@ -92,6 +92,7 @@ ignore = [
   "PLR1730",   # Replace `if` statement with `min()`
   "PLR2004",   # Magic value used in comparison
   "PLW0602",   # Using global for variable but no assignment is done
+  "PLW0603",   # Using the global statement to update a variable is discouraged
   "PLW1508",   # Invalid type for environment variable default
   "PLW1510",   # `subprocess.run` without explicit `check` argument
   "RET504",    # Unnecessary assignment before `return` statement
@@ -113,3 +114,8 @@ ignore = [
   "W291",      # Trailing whitespace
   "W293",      # Blank line contains whitespace
 ]
+
+[tool.ruff.lint.per-file-ignores]
+"tests/*" = [
+  "SLF001",    # Private member accessed (tests legitimately access module internals)
+]