[sensor] Fix codegen crash for a delta baseline given as a number (#21811)

tests/component_tests/sensor/test_delta_baseline.py

@@ -0,0 +1,16 @@
+"""Tests for the sensor delta filter baseline."""
+
+from collections.abc import Callable
+import re
+
+
+def test_delta_baseline_number(generate_main: Callable[[str], str]) -> None:
+    """A plain number baseline, including 0, becomes a lambda returning it."""
+    main_cpp = generate_main("tests/component_tests/sensor/test_delta_baseline.yaml")
+
+    # A user lambda body also carries #line directives around the return.
+    for body in (r"return 20\.0f;", r"return 0\.0f;", r"return x \* 2\.0f;"):
+        assert re.search(
+            rf"set_baseline\(\[\]\(float x\) -> float \{{[^}}]*?{body}[^}}]*\}}\);",
+            main_cpp,
+        ), body

tests/component_tests/sensor/test_delta_baseline.yaml

@@ -0,0 +1,19 @@
+esphome:
+  name: test
+
+esp8266:
+  board: d1_mini_lite
+
+sensor:
+  - platform: template
+    id: s_1
+    filters:
+      - delta:
+          max_value: 3
+          baseline: 20.0
+      - delta:
+          max_value: 3
+          baseline: 0
+      - delta:
+          max_value: 3
+          baseline: !lambda return x * 2.0f;