[ble_nus] Increase BLE MTU for NUS (#19629)

Co-authored-by: pre-commit-ci-lite[bot] <117423508+pre-commit-ci-lite[bot]@users.noreply.github.com>
Co-authored-by: J. Nick Koston <nick@home-assistant.io>

tests/component_tests/zephyr_ble_server/config/no_request.yaml

@@ -0,0 +1,8 @@
+esphome:
+  name: mtutest
+
+nrf52:
+  board: adafruit_itsybitsy_nrf52840
+  bootloader: adafruit_nrf52_sd140_v6
+
+zephyr_ble_server:

tests/component_tests/zephyr_ble_server/config/nus.yaml

@@ -0,0 +1,11 @@
+esphome:
+  name: mtutest
+
+nrf52:
+  board: adafruit_itsybitsy_nrf52840
+  bootloader: adafruit_nrf52_sd140_v6
+
+logger:
+
+ble_nus:
+  type: logs

tests/component_tests/zephyr_ble_server/config/nus_mcumgr.yaml

@@ -0,0 +1,18 @@
+esphome:
+  name: mtutest
+
+nrf52:
+  board: adafruit_itsybitsy_nrf52840
+  bootloader: adafruit_nrf52_sd140_v6
+
+logger:
+
+zephyr_ble_server:
+
+ble_nus:
+  type: logs
+
+ota:
+  - platform: zephyr_mcumgr
+    transport:
+      ble: true

tests/component_tests/zephyr_ble_server/test_l2cap_mtu.py

@@ -0,0 +1,52 @@
+"""Tests for the BLE L2CAP MTU that zephyr_ble_server writes to prj.conf."""
+
+from collections.abc import Callable
+from pathlib import Path
+
+import pytest
+
+from esphome.components.zephyr import zephyr_data
+from esphome.components.zephyr.const import KEY_PRJ_CONF
+from esphome.components.zephyr_ble_server import _emit_ble_mtu, request_ble_l2cap_mtu
+
+MTU_KEYS = (
+    "CONFIG_BT_L2CAP_TX_MTU",
+    "CONFIG_BT_BUF_ACL_TX_SIZE",
+    "CONFIG_BT_BUF_ACL_RX_SIZE",
+)
+
+
+def _mtu_values() -> tuple[int | None, ...]:
+    prj_conf = zephyr_data()[KEY_PRJ_CONF][""]
+    return tuple(prj_conf.get(key, (None,))[0] for key in MTU_KEYS)
+
+
+@pytest.mark.parametrize(
+    ("fixture", "expected"),
+    [
+        ("no_request.yaml", (None, None, None)),
+        ("nus.yaml", (247, 251, 251)),
+        # The largest request wins; only the TX buffer is capped at 251
+        ("nus_mcumgr.yaml", (498, 251, 502)),
+    ],
+)
+def test_l2cap_mtu_prj_conf(
+    fixture: str,
+    expected: tuple[int | None, ...],
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    generate_main(component_config_path(fixture))
+    assert _mtu_values() == expected
+
+
+@pytest.mark.asyncio
+async def test_request_after_emit_raises(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    generate_main(component_config_path("no_request.yaml"))
+    request_ble_l2cap_mtu(247)
+    await _emit_ble_mtu()
+    with pytest.raises(RuntimeError, match="after it was written"):
+        request_ble_l2cap_mtu(498)