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/StubWithObservableMethodCalls.ts

@@ -19,7 +19,7 @@ export abstract class StubWithObservableMethodCalls<T> {
 
 type MethodCall<T> = {
   [K in FunctionKeys<T>]: {
-    methodName: K;
-    args: T[K] extends (...args: infer A) => unknown ? A : never;
+    readonly methodName: K;
+    readonly args: T[K] extends (...args: infer A) => unknown ? A : never;
   }
 }[FunctionKeys<T>];