test: add fixture-driven suite for config_validate

Complements the inline-dict unit tests in test_config_validate.py with
10 real JSON config files under tests/utils/fixtures/ (valid sim/real/
Normal configs, one with legacy unknown keys that must NOT fail, and
six broken variants covering each distinct failure mode: missing gain,
missing Inner branch, missing pid_type, missing TempCtrl entirely,
wrong-typed gain, TempCtrl not a dict).

test_config_fixtures.py drives all of them from one CASES table
(fixture name -> expected pass/ConfigError-regex), plus a guard test
that fails if fixtures/ and CASES ever drift apart.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01F4oEE99rkKVU8L8tkzXWdG

tests/utils/test_config_fixtures.py

@@ -0,0 +1,47 @@
+import json
+import os
+import unittest
+
+from utils.config_validate import ConfigError, validate_temp_ctrl
+
+FIXTURES_DIR = os.path.join(os.path.dirname(__file__), 'fixtures')
+
+# filename -> None if the fixture should pass validation, otherwise a regex
+# matched against the raised ConfigError's message.
+CASES = {
+    'valid_sim.json': None,
+    'valid_real.json': None,
+    'valid_normal.json': None,
+    'legacy_unknown_keys.json': None,
+    'missing_outer_kp.json': r'TempCtrl\.Outer\.kp',
+    'missing_inner_hold_branch.json': r'TempCtrl\.Inner\.Hold',
+    'missing_pid_type.json': r'TempCtrl\.pid_type',
+    'missing_tempctrl_section.json': r'missing required config key: TempCtrl$',
+    'wrong_type_kp_string.json': r'TempCtrl\.Inner\.Heat\.kp',
+    'wrong_type_tempctrl_not_dict.json': r'TempCtrl must be dict',
+}
+
+
+def _load(name):
+    with open(os.path.join(FIXTURES_DIR, name)) as f:
+        return json.load(f)
+
+
+class TestConfigFixtures(unittest.TestCase):
+    def test_every_fixture_file_is_covered(self):
+        on_disk = {f for f in os.listdir(FIXTURES_DIR) if f.endswith('.json')}
+        self.assertEqual(on_disk, set(CASES), "fixtures/ and CASES have drifted apart")
+
+    def test_fixtures_match_expected_outcome(self):
+        for name, expected_error in CASES.items():
+            with self.subTest(fixture=name):
+                config = _load(name)
+                if expected_error is None:
+                    validate_temp_ctrl(config)  # must not raise
+                else:
+                    with self.assertRaisesRegex(ConfigError, expected_error):
+                        validate_temp_ctrl(config)
+
+
+if __name__ == '__main__':
+    unittest.main()