SOLR-17999: Fix the Managed Model Store Initialization in the LLM module

[ilariapet]

https://issues.apache.org/jira/browse/SOLR-17999

Description

Currently, the Text-to-Vector model store is initialised only when the corresponding LLM query parser is explicitly declared in solrconfig.xml.

As a result, users who want to use text-to-vector models exclusively at index time (via an update request processor) cannot access the model store unless a query parser is also configured, even though all required indexing components are present.

Solution

Ensure that the Text-to-Vector model store is also initialised when a text-to-vector update processor is configured.
This enables the model store to be available for index-time usage independently of any query-time configuration.

The documentation has been updated accordingly.

Tests

Tests have been added to validate the initialisation behaviour of the Text-to-Vector model store across different solrconfig.xml configurations.
They verify that the model store is correctly initialised when either the update processor or the query parser is configured, and that it is not registered otherwise, returning the expected error.

Checklist

Please review the following and check all that apply:

• 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