test(ui): Split monolithic parameter validation tests

Replaced test_parameter_validation.cpp with focused tests:
- test_parameter_metadata.cpp
- test_parameter_validator.cpp
- test_validation_result.cpp

Improves isolation and readability; updates CMake to include new files.

Author: ahojukka5

tests/unit/ui/CMakeLists.txt

@@ -5,5 +5,7 @@
 target_sources(openpfc-tests PRIVATE
   test_ui_errors.cpp
   test_toml_to_json.cpp
-  test_parameter_validation.cpp
+  test_parameter_metadata.cpp
+  test_parameter_validator.cpp
+  test_validation_result.cpp
 )