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/application/Parser/Script/Validation/CodeValidator.spec.ts

@@ -4,7 +4,7 @@ import { CodeValidationRuleStub } from '@tests/unit/shared/Stubs/CodeValidationR
 import { itEachAbsentCollectionValue, itEachAbsentStringValue } from '@tests/unit/shared/TestCases/AbsentTests';
 import { itIsSingleton } from '@tests/unit/shared/TestCases/SingletonTests';
 import { ICodeLine } from '@/application/Parser/Script/Validation/ICodeLine';
-import { IInvalidCodeLine } from '@/application/Parser/Script/Validation/ICodeValidationRule';
+import { ICodeValidationRule, IInvalidCodeLine } from '@/application/Parser/Script/Validation/ICodeValidationRule';
 
 describe('CodeValidator', () => {
   describe('instance', () => {
@@ -23,10 +23,10 @@ describe('CodeValidator', () => {
         const act = () => sut.throwIfInvalid(code, [new CodeValidationRuleStub()]);
         // assert
         expect(act).to.not.throw();
-      });
+      }, { excludeNull: true, excludeUndefined: true });
     });
     describe('throws if rules are empty', () => {
-      itEachAbsentCollectionValue((absentValue) => {
+      itEachAbsentCollectionValue<ICodeValidationRule>((absentValue) => {
         // arrange
         const expectedError = 'missing rules';
         const rules = absentValue;
@@ -35,7 +35,7 @@ describe('CodeValidator', () => {
         const act = () => sut.throwIfInvalid('code', rules);
         // assert
         expect(act).to.throw(expectedError);
-      });
+      }, { excludeUndefined: true, excludeNull: true });
     });
     describe('splits lines as expected', () => {
       it('supports all line separators', () => {

tests/unit/application/Parser/Script/Validation/Rules/NoDuplicatedLines.spec.ts

@@ -1,24 +1,10 @@
-import { describe, expect } from 'vitest';
-import { itEachAbsentObjectValue } from '@tests/unit/shared/TestCases/AbsentTests';
+import { describe } from 'vitest';
 import { NoDuplicatedLines } from '@/application/Parser/Script/Validation/Rules/NoDuplicatedLines';
 import { LanguageSyntaxStub } from '@tests/unit/shared/Stubs/LanguageSyntaxStub';
 import { IInvalidCodeLine } from '@/application/Parser/Script/Validation/ICodeValidationRule';
 import { testCodeValidationRule } from './CodeValidationRuleTestRunner';
 
 describe('NoDuplicatedLines', () => {
-  describe('ctor', () => {
-    describe('throws if syntax is missing', () => {
-      itEachAbsentObjectValue((absentValue) => {
-        // arrange
-        const expectedError = 'missing syntax';
-        const syntax = absentValue;
-        // act
-        const act = () => new NoDuplicatedLines(syntax);
-        // assert
-        expect(act).to.throw(expectedError);
-      });
-    });
-  });
   describe('analyze', () => {
     testCodeValidationRule([
       {

tests/unit/application/Parser/Script/Validation/Rules/NoEmptyLines.spec.ts

@@ -23,12 +23,6 @@ describe('NoEmptyLines', () => {
         expected: [2, 4].map((index) => ({ index, error: 'Empty line' })),
         sut: new NoEmptyLines(),
       },
-      {
-        testName: 'shows error for undefined and null lines',
-        codeLines: ['first line', undefined, 'third line', null],
-        expected: [2, 4].map((index) => ({ index, error: 'Empty line' })),
-        sut: new NoEmptyLines(),
-      },
       {
         testName: 'shows error for whitespace-only lines',
         codeLines: ['first line', '  ', 'third line'],