fix(threat-intel): restore export on collapseWhitespace

Adapters in the RSS/STIX/TAXII (elastic#287205) and vendor_api (elastic#287206) slices import
collapseWhitespace from content/text.ts. The content-parsing rewrite made it a
private const, which breaks those imports once this PR merges. It's a plain
string utility with real external callers, not part of the parsing-architecture
change, so restore the export.

Author: stephmilovic

x-pack/solutions/security/plugins/security_solution/server/threat_intel/content/text.ts

@@ -70,7 +70,7 @@ interface SectionState {
 }
 
 /** Collapses visible whitespace into the storage representation. */
-const collapseWhitespace = (input: string): string => input.replace(/\s+/g, ' ').trim();
+export const collapseWhitespace = (input: string): string => input.replace(/\s+/g, ' ').trim();
 
 const pushChildren = (stack: SerializeStep[], element: Element): void => {
   const children = Array.from(element.childNodes);