Fix file fulltext being wiped by partial Solr updates

[luis100]

Summary

• FileCollection's fulltext field was stored=false. Solr's atomic/partial update ({"set": ...}) reconstructs the whole document from its stored/docValues fields before re-submitting it, so any field that's neither stored nor docValues is silently dropped in that reconstruction - fulltext was being wiped on any partial update to an IndexedFile document (e.g. AIP move, which patches FILE_ANCESTORS, or AIP permission propagation), taking the fulltext→search copyField's terms down with it, so basic/default search on the file was affected too, not just explicit fulltext: queries.
• Fix: drop .setStored(false) so the field uses Field's documented implicit default (stored=true), letting Solr correctly preserve it across partial updates. Also makes fulltext's raw value retrievable via fl=, which previously silently returned nothing for this field.

Fixes #3733

Test plan

• Compiles (single-field schema definition change, no other code paths touched)
• Against a throwaway Solr instance bootstrapped from scratch: confirm the file collection reports stored=true for fulltext, and that fl=fulltext returns real content after indexing a file with known text
• Confirm fulltext:<term> and default/basic search both still match a file after triggering a partial update on it (e.g. moving its parent AIP)

Note for anyone applying this against an already-bootstrapped Solr collection: SolrBootstrapUtils.bootstrapCollection only logs a warning for an existing field whose attributes no longer match - it never issues a replace-field - so this won't retroactively apply to a live collection without either a schema replace-field call or a from-scratch re-bootstrap, followed by a full File reindex to repopulate fulltext for already-indexed documents.