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_context.py

@@ -0,0 +1,20 @@
+import unittest
+from unittest.mock import patch
+
+from amc_server.context import _resolve_zellij_bin
+
+
+class ContextTests(unittest.TestCase):
+    def test_resolve_zellij_bin_prefers_which(self):
+        with patch("amc_server.context.shutil.which", return_value="/custom/bin/zellij"):
+            self.assertEqual(_resolve_zellij_bin(), "/custom/bin/zellij")
+
+    def test_resolve_zellij_bin_falls_back_to_default_name(self):
+        with patch("amc_server.context.shutil.which", return_value=None), patch(
+            "amc_server.context.Path.exists", return_value=False
+        ), patch("amc_server.context.Path.is_file", return_value=False):
+            self.assertEqual(_resolve_zellij_bin(), "zellij")
+
+
+if __name__ == "__main__":
+    unittest.main()