SOLR-18510: Port AssertTool to picocli

[serhiy-bzhezytskyy]

https://issues.apache.org/jira/browse/SOLR-18510

Description

Adds the picocli code path for bin/solr assert, next to the commons-cli one. test_auth.bats uses assert to wait for startup, so it could not join the picocli suite until now.

Solution

AssertTool gets its annotated options and callTool() and is registered in SolrCLI; the commons-cli path is unchanged. Written with Claude Code.

Tests

AssertToolPicocliTest runs the existing AssertToolTest through picocli. test_auth.bats passes under both parsers and is added to integrationTestsPicocli; check -x test is clean (rat skipped).

Checklist

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide
• I have added a changelog entry for my change

[janhoy]

LGTM