[espnow] Keep constant send payloads in shared flash tables (#20271)

tests/component_tests/espnow/config/payload_tables.yaml

@@ -0,0 +1,22 @@
+---
+esphome:
+  name: test
+  on_boot:
+    then:
+      - espnow.send:
+          address: 11:22:33:44:55:66
+          data: [0x01, 0x02, 0x03]
+      - espnow.broadcast:
+          data: [0x01, 0x02, 0x03]
+      - espnow.broadcast: "OK"
+      - espnow.broadcast:
+          data: !lambda return {0x09};
+
+esp32:
+  board: esp32dev
+
+wifi:
+  ssid: test
+  password: password1
+
+espnow:

tests/component_tests/espnow/test_payload_tables.py

@@ -0,0 +1,23 @@
+"""Tests for ESP-NOW constant send payloads in shared flash tables."""
+
+from collections.abc import Callable
+from pathlib import Path
+import re
+
+
+def test_constant_payloads_share_tables(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    """Equal payloads share one table; lambdas stay templates."""
+    main_cpp = generate_main(component_config_path("payload_tables.yaml"))
+
+    tables = dict(
+        re.findall(
+            r"static constexpr uint8_t (\w+)\[\] PROGMEM = (\{[^}]*\});", main_cpp
+        )
+    )
+    assert sorted(tables.values()) == sorted(["{0x01, 0x02, 0x03}", "{0x4F, 0x4B}"])
+    shared = next(k for k, v in tables.items() if v == "{0x01, 0x02, 0x03}")
+    assert main_cpp.count(f"set_data_static({shared}, 3);") == 2
+    assert "set_data_template(" in main_cpp