Implemented the next missing parity slice around session-scoped environment variables.

tests/test_session_env_vars.py

@@ -0,0 +1,63 @@
+"""Tests for ``src/session_env_vars.py``."""
+
+from __future__ import annotations
+
+import unittest
+
+from src.session_env_vars import (
+    clear_session_env_vars,
+    delete_session_env_var,
+    get_session_env_vars,
+    set_session_env_var,
+)
+
+
+class SessionEnvVarsTest(unittest.TestCase):
+    def setUp(self) -> None:
+        clear_session_env_vars()
+
+    def tearDown(self) -> None:
+        clear_session_env_vars()
+
+    def test_starts_empty(self) -> None:
+        self.assertEqual(dict(get_session_env_vars()), {})
+
+    def test_set_and_get(self) -> None:
+        set_session_env_var('FOO', 'bar')
+        self.assertEqual(get_session_env_vars()['FOO'], 'bar')
+
+    def test_set_overwrites_existing(self) -> None:
+        set_session_env_var('FOO', 'one')
+        set_session_env_var('FOO', 'two')
+        self.assertEqual(get_session_env_vars()['FOO'], 'two')
+
+    def test_delete_removes(self) -> None:
+        set_session_env_var('FOO', 'bar')
+        delete_session_env_var('FOO')
+        self.assertNotIn('FOO', get_session_env_vars())
+
+    def test_delete_missing_is_noop(self) -> None:
+        delete_session_env_var('NEVER_SET')
+        self.assertEqual(dict(get_session_env_vars()), {})
+
+    def test_clear_drops_everything(self) -> None:
+        set_session_env_var('A', '1')
+        set_session_env_var('B', '2')
+        clear_session_env_vars()
+        self.assertEqual(dict(get_session_env_vars()), {})
+
+    def test_returned_mapping_is_read_only(self) -> None:
+        set_session_env_var('FOO', 'bar')
+        view = get_session_env_vars()
+        with self.assertRaises(TypeError):
+            view['FOO'] = 'mutated'  # type: ignore[index]
+
+    def test_view_reflects_subsequent_mutations(self) -> None:
+        view = get_session_env_vars()
+        self.assertNotIn('FOO', view)
+        set_session_env_var('FOO', 'bar')
+        self.assertEqual(view['FOO'], 'bar')
+
+
+if __name__ == '__main__':
+    unittest.main()