Type-safe null handling

Use TypeScript’s type system and modern JavaScript features to prevent null reference errors.

TypeScript type safety:

• Prefer never over undefined for properties that shouldn’t have values
• Use unknown instead of any when working with data of uncertain structure
• Validate data with zod schemas instead of using type assertions (as)
• Use proper type narrowing with type guards

Modern JavaScript null checks:

• Use nullish coalescing (??) for default values: ```typescript // Instead of this: const citations = response.message.citations ? response.message.citations : [];

// Do this: const citations = response.message.citations ?? [];

- Use optional chaining (`?.`) for accessing properties on potentially undefined objects:
```typescript
// Instead of this:
const contentType = requestClone.headers.get('content-type') || '';
if (contentType.startsWith('multipart/form-data')) { /*...*/ }

// Do this:
const contentType = requestClone.headers.get('content-type');
if (contentType?.startsWith('multipart/form-data')) { /*...*/ }

Following these patterns leads to more robust, self-documenting code and fewer runtime errors from unexpected null/undefined values.