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/cmock.yml

@@ -0,0 +1,7 @@
+:cmock:
+  :mock_prefix: Mock
+  :when_no_prototypes: nothing
+  :weak: __attribute__((weak))
+  :plugins:
+    - :ignore_arg
+    - :expect_any_args