feat(modem): Add end2end host tests with host modem_sim

[david-cermak]

Add end-to-end host test with modem simulator

Adds a new host_test_app under components/esp_modem/test/ that exercises the esp_modem AT command path end-to-end on a Linux host, using two cooperating processes connected over a TCP socket.

The test consists of two parts:

modem_sim

A standalone C++ TCP server (no IDF dependency) that emulates a SIM7600-like modem. It listens on localhost, accepts connections, and responds to AT commands (AT+CSQ, AT+CGMM, AT+CGSN, AT+CIMI, AT+COPS?, AT+CPIN?, +++, etc.). Responses can be delivered in configurable batches (batch_size / batch_delay_ms) to simulate fragmented serial data arriving in chunks rather than complete lines.

test_app

An IDF Linux application (Catch2) that creates a VFS socket DTE via vfs_create_socket, instantiates a SIM7600 DCE through the standard esp_modem factory API, and runs AT command tests against the simulator. Seven test cases cover set_command_mode, get_signal_quality, get_module_name, get_operator_name, get_imsi, get_imei, and read_pin.

A run_test.sh harness script orchestrates both processes: builds the simulator if needed, starts it in the background, runs the test binary with a configurable timeout, and cleans up.

esp_netif_linux stub mode

To avoid requiring a lwIP download for AT-only testing, the esp_netif_linux component's CMakeLists.txt now checks whether LWIP_PATH is set. If not, it registers a minimal stub (esp_netif_stub.c) providing only the headers and no-op implementations of esp_netif_new/esp_netif_destroy/esp_netif_receive, then returns early. The full lwIP+TUN build path is unchanged when LWIP_PATH is defined. This makes the existing host_test and linux_modem example unaffected while enabling lightweight test builds.

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Note

Medium Risk
Introduces new CI-executed host tests and a conditional esp_netif_linux stub path, which could affect Linux build/link behavior when lwIP isn’t configured. Runtime impact should be limited to the Linux port and test scaffolding.

Overview
Adds a new components/esp_modem/test/host_test_app end-to-end Linux host test that runs Catch2-based AT-command tests against a new standalone TCP modem_sim process, with a run_test.sh harness and CI configs for the Linux target.

Updates CI (modem__build-host-tests.yml) to build and execute this host test across multiple IDF versions and run it under different response chunking settings to simulate fragmented modem I/O.

Makes Linux builds less dependent on networking by adding an esp_netif_linux stub mode when LWIP_PATH isn’t present (registering esp_netif_stub.c), and introduces a small FreeRTOS portmacro.h shim for Linux critical-section/spinlock APIs.

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