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/shared/Stubs/CodeValidatorStub.ts

@@ -20,7 +20,7 @@ export class CodeValidatorStub implements ICodeValidator {
   }
 
   public assertHistory(expected: {
-    validatedCodes: readonly string[],
+    validatedCodes: readonly (string | undefined)[],
     rules: readonly Constructible<ICodeValidationRule>[],
   }) {
     expect(this.callHistory).to.have.lengthOf(expected.validatedCodes.length);