[udp] Keep constant write payloads in shared flash tables (#20268)

tests/component_tests/udp/config/udp_write.yaml

@@ -0,0 +1,22 @@
+esphome:
+  name: test
+
+esp8266:
+  board: d1_mini
+
+wifi:
+  ssid: test
+  password: password1
+
+udp:
+  addresses:
+    - 10.0.0.1
+
+button:
+  - platform: template
+    name: send
+    on_press:
+      - udp.write: [0xAA, 0x55, 0x01, 0x02]
+      - udp.write: [0xAA, 0x55, 0x01, 0x02]
+      - udp.write: ""
+      - udp.write: !lambda return {0x10, 0x20};

tests/component_tests/udp/test_udp_write.py

@@ -0,0 +1,17 @@
+"""Tests for the udp.write action codegen."""
+
+from collections.abc import Callable
+from pathlib import Path
+
+
+def test_write_payloads_share_a_progmem_table(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    """Identical constant payloads share one flash table; an empty one needs none."""
+    main_cpp = generate_main(component_config_path("udp_write.yaml"))
+
+    assert main_cpp.count("udp_data[] PROGMEM = {0xAA, 0x55, 0x01, 0x02};") == 1
+    assert main_cpp.count("->set_data_static(udp_data, 4);") == 2
+    assert "->set_data_static(nullptr, 0);" in main_cpp
+    assert "->set_data_template(" in main_cpp