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/CategoryStub.ts

@@ -1,5 +1,6 @@
 import { BaseEntity } from '@/infrastructure/Entity/BaseEntity';
 import { ICategory, IScript } from '@/domain/ICategory';
+import { RecommendationLevel } from '@/domain/RecommendationLevel';
 import { ScriptStub } from './ScriptStub';
 
 export class CategoryStub extends BaseEntity<number> implements ICategory {
@@ -26,37 +27,44 @@ export class CategoryStub extends BaseEntity<number> implements ICategory {
     ];
   }
 
-  public withScriptIds(...scriptIds: string[]): CategoryStub {
+  public withScriptIds(...scriptIds: string[]): this {
     return this.withScripts(
       ...scriptIds.map((id) => new ScriptStub(id)),
     );
   }
 
-  public withScripts(...scripts: IScript[]): CategoryStub {
+  public withScripts(...scripts: IScript[]): this {
     for (const script of scripts) {
       this.withScript(script);
     }
     return this;
   }
 
-  public withCategories(...categories: ICategory[]): CategoryStub {
+  public withMandatoryScripts(): this {
+    return this
+      .withScript(new ScriptStub(`[${CategoryStub.name}] script-1`).withLevel(RecommendationLevel.Standard))
+      .withScript(new ScriptStub(`[${CategoryStub.name}] script-2`).withLevel(RecommendationLevel.Strict))
+      .withScript(new ScriptStub(`[${CategoryStub.name}] script-3`).withLevel(undefined));
+  }
+
+  public withCategories(...categories: ICategory[]): this {
     for (const category of categories) {
       this.withCategory(category);
     }
     return this;
   }
 
-  public withCategory(category: ICategory): CategoryStub {
+  public withCategory(category: ICategory): this {
     this.subCategories.push(category);
     return this;
   }
 
-  public withScript(script: IScript): CategoryStub {
+  public withScript(script: IScript): this {
     this.scripts.push(script);
     return this;
   }
 
-  public withName(categoryName: string) {
+  public withName(categoryName: string): this {
     this.name = categoryName;
     return this;
   }