[types] - fix: ensure non-optional string for content type check

 - Removed unnecessary optionality of the `contentType` parameter in `isTextExtractionSupportedContentType` function
 - Enforce the function to always receive a string argument, aligning with supported content types check

types/src/shared/text_extraction.ts

@@ -212,7 +212,7 @@ export class TextExtraction {
 }
 
 export function isTextExtractionSupportedContentType(
-  contentType: string | undefined
+  contentType: string
 ): contentType is SupportedContentTypes {
   return supportedContentTypes.includes(contentType as SupportedContentTypes);
 }