register_fprime_config: STATIC enforcement checks `SOURCE` instead of `SOURCES`

Problem Description

fprime_add_config_build_target is meant to force a configuration module to STATIC when it supplies sources or autocoder inputs, but it tests the wrong directive name:

# cmake/API.cmake
function(fprime_add_config_build_target)
    set(ARGN_PASS ${ARGN})
    # Ensure library is STATIC when supplying SOURCE or AUTOCODER_INPUTS
    if (SOURCE IN_LIST ARGN_PASS OR AUTOCODER_INPUTS IN_LIST ARGN_PASS)

The directive users pass is SOURCES, so the first test never matches. A config module with SOURCES but no AUTOCODER_INPUTS and no explicit STATIC is therefore not forced static and follows BUILD_SHARED_LIBS like any other library. In-tree config modules all have FPP inputs, so AUTOCODER_INPUTS masks this; present since #3642 (378a7a3e6f).

How to Reproduce

1. Register a config module with SOURCES only (no AUTOCODER_INPUTS, no STATIC/INTERFACE).
2. Generate with -DBUILD_SHARED_LIBS=ON.
3. Observe the target is a shared library.

Expected Behavior

The module is built STATIC, as the docstring states. Fix: SOURCE -> SOURCES on that line, plus a cmake/test case for a SOURCES-only config module under BUILD_SHARED_LIBS=ON.