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.

src/infrastructure/RuntimeEnvironment/RuntimeEnvironment.ts

@@ -37,7 +37,7 @@ export class RuntimeEnvironment implements IRuntimeEnvironment {
   }
 }
 
-function getUserAgent(window: Partial<Window>): string {
+function getUserAgent(window: Partial<Window>): string | undefined {
   return window?.navigator?.userAgent;
 }