[WIP] Add test coverage for Elasticsearch plugin functionality

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Techdebt]: Advanced Search: Test coverage within Elasticsearch plugin specifically</issue_title>
<issue_description>Topics to cover:

• versioned index, including migrations function
• index mapping
• query mapping
• highlight extractor
• multi-match classifier, phrase selection classifier</issue_description>

<agent_instructions>The entire plugin crate is located at src/infra/search/elasticsearch.
It does not have any tests yet.
Decide yourself whether to add unit tests (#[cfg(test)]) or integration tests.
If using intergration tests, follow good examples in the project, such as creating harness structs to encapsulate dependencies and low level steps, cut off non-critical dependencies with dummies, mocks, or at least in-memory repositories.
</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [Techdebt]: Advanced Search: Test coverage within Elasticsearch plugin specifically #1566

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