[network] Add tcp_send_buffer option

tests/component_tests/network/config/tcp_send_buffer.yaml

@@ -0,0 +1,14 @@
+esphome:
+  name: test
+
+esp32:
+  board: esp32dev
+  framework:
+    type: esp-idf
+
+wifi:
+  ssid: "test_ssid"
+  password: "test_password"
+
+network:
+  tcp_send_buffer: 32kB

tests/component_tests/network/config/tcp_send_buffer_high_perf.yaml

@@ -0,0 +1,15 @@
+esphome:
+  name: test
+
+esp32:
+  board: esp32dev
+  framework:
+    type: esp-idf
+
+wifi:
+  ssid: "test_ssid"
+  password: "test_password"
+
+network:
+  enable_high_performance: true
+  tcp_send_buffer: 16384

tests/component_tests/network/test_tcp_send_buffer.py

@@ -0,0 +1,66 @@
+"""Tests for the ``network: tcp_send_buffer:`` option.
+
+The option sets lwIP's per-socket TCP send buffer
+(CONFIG_LWIP_TCP_SND_BUF_DEFAULT) on ESP-IDF. The stock default (5744 bytes)
+stalls bursty senders such as a Bluetooth proxy streaming GATT notifications;
+until now the only way to raise it was the all-or-nothing
+``enable_high_performance`` bundle.
+"""
+
+from collections.abc import Callable
+from pathlib import Path
+
+import pytest
+from voluptuous import Invalid
+
+from esphome import config_validation as cv
+from esphome.components.esp32.const import KEY_VARIANT, VARIANT_ESP32
+from esphome.components.network import CONFIG_SCHEMA
+from esphome.const import KEY_FRAMEWORK_VERSION, PlatformFramework
+from esphome.core import CORE
+from tests.component_tests.types import SetCoreConfigCallable
+
+
+def _sdkconfig_option(name: str) -> int | None:
+    from esphome.components.esp32.const import KEY_ESP32, KEY_SDKCONFIG_OPTIONS
+
+    return CORE.data[KEY_ESP32][KEY_SDKCONFIG_OPTIONS].get(name)
+
+
+def test_tcp_send_buffer_sets_sdkconfig(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    generate_main(component_config_path("tcp_send_buffer.yaml"))
+    assert _sdkconfig_option("CONFIG_LWIP_TCP_SND_BUF_DEFAULT") == 32000
+
+
+def test_tcp_send_buffer_overrides_high_performance(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    """An explicit size wins over the high performance bundle's 65534."""
+    generate_main(component_config_path("tcp_send_buffer_high_perf.yaml"))
+    assert _sdkconfig_option("CONFIG_LWIP_TCP_SND_BUF_DEFAULT") == 16384
+
+
+@pytest.mark.parametrize("value", ["1kB", "128kB"])
+def test_out_of_range_rejected(
+    set_core_config: SetCoreConfigCallable, value: str
+) -> None:
+    set_core_config(
+        PlatformFramework.ESP32_IDF,
+        core_data={KEY_FRAMEWORK_VERSION: cv.Version(5, 5, 5)},
+        platform_data={KEY_VARIANT: VARIANT_ESP32},
+    )
+    with pytest.raises(Invalid):
+        CONFIG_SCHEMA({"tcp_send_buffer": value})
+
+
+def test_rejected_on_esp8266(set_core_config: SetCoreConfigCallable) -> None:
+    set_core_config(
+        PlatformFramework.ESP8266_ARDUINO,
+        core_data={KEY_FRAMEWORK_VERSION: cv.Version(3, 1, 2)},
+    )
+    with pytest.raises(Invalid, match="esp32"):
+        CONFIG_SCHEMA({"tcp_send_buffer": "32kB"})