config: fail fast on missing TempCtrl PID gains

Config access was unchecked dict[key] everywhere, so a missing/
misnamed gain (we've hit this once already: config.json.sim's gain/
Model nesting mismatch) surfaced as a bare KeyError from deep inside
some component's __init__ or first process() tick, with no indication
which config key was wrong.

utils/config_validate.py adds a small stdlib-only dotted-path checker
(no schema library - matches this project's stdlib-first leaning) and
validate_temp_ctrl(), scoped to the section that actually caused an
incident: TempCtrl.pid_type plus every kp/ki/kd/kt gain under
TempCtrl.Outer and TempCtrl.Inner.{Heat,Hold,Cool}. Called once in
server/brewpi.py right after json.load(), before any factory runs.
Optional-with-default keys (Outer.y_hold_min, Inner.Hold.yi_max,
Thresholds) stay optional here too.

Heater/Stirrer/TempSensor/Pot sections and unknown-key warnings are
deliberately left for later - see components/pid/TODO.md.

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

tests/utils/test_config_validate.py

@@ -0,0 +1,72 @@
+import unittest
+
+from utils.config_validate import ConfigError, require, validate_temp_ctrl
+
+
+def _valid_temp_ctrl_config():
+    gains = {"kp": 0.08, "ki": 0.02, "kd": 0.0, "kt": 1.5}
+    return {
+        "TempCtrl": {
+            "pid_type": "Smith",
+            "Outer": dict(gains),
+            "Inner": {
+                "Heat": dict(gains),
+                "Hold": dict(gains),
+                "Cool": dict(gains),
+            },
+        }
+    }
+
+
+class TestRequire(unittest.TestCase):
+    def test_returns_value_when_present(self):
+        self.assertEqual(require({"a": {"b": 1}}, "a.b"), 1)
+
+    def test_raises_on_missing_leaf(self):
+        with self.assertRaises(ConfigError):
+            require({"a": {}}, "a.b")
+
+    def test_raises_on_missing_intermediate(self):
+        with self.assertRaises(ConfigError):
+            require({}, "a.b")
+
+    def test_error_names_full_path_up_to_missing_key(self):
+        with self.assertRaisesRegex(ConfigError, r"a\.b"):
+            require({"a": {}}, "a.b.c")
+
+    def test_raises_on_wrong_type(self):
+        with self.assertRaises(ConfigError):
+            require({"a": "not a number"}, "a", type=(int, float))
+
+    def test_accepts_matching_type(self):
+        self.assertEqual(require({"a": 1.5}, "a", type=(int, float)), 1.5)
+
+
+class TestValidateTempCtrl(unittest.TestCase):
+    def test_valid_config_passes(self):
+        validate_temp_ctrl(_valid_temp_ctrl_config())
+
+    def test_missing_gain_raises(self):
+        config = _valid_temp_ctrl_config()
+        del config["TempCtrl"]["Inner"]["Hold"]["kp"]
+        with self.assertRaisesRegex(ConfigError, r"TempCtrl\.Inner\.Hold\.kp"):
+            validate_temp_ctrl(config)
+
+    def test_missing_pid_type_raises(self):
+        config = _valid_temp_ctrl_config()
+        del config["TempCtrl"]["pid_type"]
+        with self.assertRaises(ConfigError):
+            validate_temp_ctrl(config)
+
+    def test_optional_keys_not_required(self):
+        config = _valid_temp_ctrl_config()
+        # Outer.y_hold_min, Inner.Hold.yi_max, and the whole Thresholds
+        # section all have their own defaults elsewhere and must stay
+        # optional here too.
+        validate_temp_ctrl(config)
+        self.assertNotIn("y_hold_min", config["TempCtrl"]["Outer"])
+        self.assertNotIn("Thresholds", config["TempCtrl"])
+
+
+if __name__ == '__main__':
+    unittest.main()