feat: implement port find/probe and add platform description

Implement cel_serial_find_elrs_port() which enumerates serial ports
and matches descriptions against ELRS-related keywords (CP210, CH340,
FTDI, etc.). Implement cel_serial_open_probe() to try multiple baud
rates in order.

Add cel_serial_platform_get_description() for Windows (SetupAPI)
and POSIX (sysfs fallback). Wire setupapi into the Windows build.
Update serial tests with CMock expectations for the new functions.

tests/CMakeLists.txt

@@ -18,6 +18,7 @@ function(cmock_generate_mock target header)
     add_custom_command(
         OUTPUT  "${mock_src}" "${mock_hdr}"
         COMMAND "${RUBY_EXECUTABLE}" "${CMOCK_SCRIPT}"
+                "-o" "${CMAKE_CURRENT_SOURCE_DIR}/cmock.yml"
                 "--mock_path=${MOCK_GEN_DIR}"
                 "${header}"
         DEPENDS "${header}"