Refactor to enforce strictNullChecks

This commit applies `strictNullChecks` to the entire codebase to improve
maintainability and type safety. Key changes include:

- Remove some explicit null-checks where unnecessary.
- Add necessary null-checks.
- Refactor static factory functions for a more functional approach.
- Improve some test names and contexts for better debugging.
- Add unit tests for any additional logic introduced.
- Refactor `createPositionFromRegexFullMatch` to its own function as the
  logic is reused.
- Prefer `find` prefix on functions that may return `undefined` and
  `get` prefix for those that always return a value.

tests/unit/infrastructure/SystemOperations/WindowInjectedSystemOperations.spec.ts

@@ -10,7 +10,7 @@ describe('WindowInjectedSystemOperations', () => {
       itEachAbsentObjectValue((absentValue) => {
         // arrange
         const expectedError = 'missing window';
-        const window: WindowVariables = absentValue;
+        const window: WindowVariables = absentValue as never;
         // act
         const act = () => getWindowInjectedSystemOperations(window);
         // assert
@@ -23,7 +23,7 @@ describe('WindowInjectedSystemOperations', () => {
         const expectedError = 'missing system';
         const absentSystem = absentValue;
         const window: Partial<WindowVariables> = {
-          system: absentSystem,
+          system: absentSystem as never,
         };
         // act
         const act = () => getWindowInjectedSystemOperations(window);