test(server): add unit tests for context, control, and state mixins

Adds comprehensive test coverage for the amc_server package:

- test_context.py: Tests _resolve_zellij_bin preference order (which
  first, then fallback to bare name)
- test_control.py: Tests SessionControlMixin including two-step Enter
  injection with configurable delay, freeform response handling,
  plugin inject with explicit session/pane targeting, and unsafe
  fallback behavior
- test_state.py: Tests StateMixin zellij session parsing with the
  resolved binary path

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/test_state.py

@@ -0,0 +1,37 @@
+import subprocess
+import unittest
+from unittest.mock import patch
+
+import amc_server.mixins.state as state_mod
+from amc_server.mixins.state import StateMixin
+
+
+class DummyStateHandler(StateMixin):
+    pass
+
+
+class StateMixinTests(unittest.TestCase):
+    def test_get_active_zellij_sessions_uses_resolved_binary_and_parses_output(self):
+        handler = DummyStateHandler()
+        state_mod._zellij_cache["sessions"] = None
+        state_mod._zellij_cache["expires"] = 0
+
+        completed = subprocess.CompletedProcess(
+            args=[],
+            returncode=0,
+            stdout="infra [created 1h ago]\nwork\n",
+            stderr="",
+        )
+
+        with patch.object(state_mod, "ZELLIJ_BIN", "/opt/homebrew/bin/zellij"), patch(
+            "amc_server.mixins.state.subprocess.run", return_value=completed
+        ) as run_mock:
+            sessions = handler._get_active_zellij_sessions()
+
+        self.assertEqual(sessions, {"infra", "work"})
+        args = run_mock.call_args.args[0]
+        self.assertEqual(args, ["/opt/homebrew/bin/zellij", "list-sessions", "--no-formatting"])
+
+
+if __name__ == "__main__":
+    unittest.main()