issue 1788 slice 2: SolrJ 10 compile cutover + unit tests (system/metadata primary) #1997

Parent

Part of #1788. Depends on inventory from #1996 (slice 1).

Goal

On a feature branch from main, bump SolrJ to 10.x only as part of real code refactor (not Dependabot-only), fix compile breaks in system (and WebUI if needed), and add unit tests around client construction / error handling. Do not merge a bare solr.version edit.

Scope

1. Branch from main (e.g. fix/issue-1788-solrj-10-cutover).
2. Set root solr.version to current 10.x patch (or 10.0.0+) with accompanying Java changes.
3. Refactor PSSolrDeliveryHandler#getClient:
   • Replace removed HttpSolrClient (HttpClient-based) with SolrJ 10 supported client (JDK and/or solr-solrj-jetty as inventory directs).
   • Preserve multipart / extract upload behavior equivalent to setUseMultiPartPost(true).
   • Revisit cloud path: ZK list vs Solr URLs; default collection on builder; document if cloud path remains supported given product POM excludes ZK today.
4. Fix any import/package moves (SolrQuery, client packages, etc.).
5. Unit tests:
   • Mock SolrClient where practical for add/delete/commit paths.
   • Cover standalone vs cloud construction branches without a live Solr.
   • Keep SolrConfigLoaderTest green; expand as needed.
6. Per-module gates: cd system && …/mvnw clean install (and WebUI if touched). Spotless apply then check. Erlang pre-commit on authored code.

Out of scope

• Full DTS live smoke (slice 4)
• Dependabot build(deps): bump org.apache.solr:solr-solrj from 9.10.1 to 10.0.0 #1777 close (slice 5)
• Live cluster credentials
• Java 8 line

Acceptance

• system (and any other touched module) clean install green on JDK 21
• No HttpClient-based removed SolrClient APIs remain in product code
• Unit tests for construction / key update paths
• PR against main linked from chore(deps): upgrade Apache Solr / SolrJ from 9.10.1 to 10.x (needs code refactor) #1788 (not a Dependabot-only diff)

Sequencing

After #1996. May land with or before packaging slice 3 if POM optional modules are required for compile.