[api] Check the encoded end against the reserved size under ESPHOME_DEBUG_API

The fixed32 store helper moves to a private section since it neither bounds checks nor
advances the cursor, its comment describes the path each target takes, the generated
file scan flags any ProtoEncode call that does not assign the cursor, and StateWaiter
timeouts can carry a label so gathered waits are told apart.

tests/integration/test_api_encode_boundaries.py

@@ -64,13 +64,15 @@ async def test_api_encode_boundaries(
                     isinstance(s, SensorState)
                     and s.key == sensor.key
                     and s.state == 12.5
-                )
+                ),
+                label="sensor 12.5",
             ),
             waiter.expect(
                 lambda s: (
                     isinstance(s, TextSensorState)
                     and s.key == text.key
                     and s.state == "y" * 200
-                )
+                ),
+                label="text 200 x y",
             ),
         )