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/Context/State/Selection/UserSelectionTestRunner.ts

@@ -80,7 +80,7 @@ function expectSameScripts(actual: readonly SelectedScript[], expected: readonly
       `Scripts with different statuses:\n${
         scriptsWithDifferentStatus
           .map((s) => `[id: ${s.id}, actual status: ${s.revert}, `
-            + `expected status: ${expected.find((existing) => existing.id === s.id).revert}]`)
+            + `expected status: ${expected.find((existing) => existing.id === s.id)?.revert ?? 'unknown'}]`)
           .join(' , ')
       }`,
     );