Reduce fulltext match boost

[JPrevost]

Why are these changes being introduced:

• Fulltext fields are very large blocks of text compared to other metadata-only fields.
• Reducing the boost for fulltext fields ensures that matches in smaller, more specific metadata fields are not overshadowed by matches in large text blocks.
• We had evidence that relevance scores as measured by Quepid were being negatively affected under some conditions.

Relevant ticket(s):

• https://mitlibraries.atlassian.net/browse/USE-723

How does this address that need:

• Reduced the boost for fulltext fields in the search query.

Developer

• All new ENV is documented in README
• All new ENV has been added to Heroku Pipeline, Staging and Prod
• ANDI or Wave has been run in accordance to
  our guide and
  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)
• Stakeholder approval has been confirmed (or is not needed)

Code Reviewer

• The commit message is clear and follows our guidelines
  (not just this pull request message)
• There are appropriate tests covering any new functionality
• The documentation has been updated or is unnecessary
• The changes have been verified
• New dependencies are appropriate or there were no changes

Requires database migrations?

NO

Includes new or updated dependencies?

NO

[qltysh]

All good ✅

[Copilot]

🟢 Approval recommended

The boost change and corresponding test update are reviewed with no unresolved issues.

0 open findings

What changed in this PR

Reduces the relevance boost for fulltext matches so more specific metadata fields rank higher.

Changes:

• Applies a 0.1 boost to fulltext.
• Updates the query-builder test expectation.

File	Summary
test/​models/​lexical_query_builder_test.rb	Verifies the updated field boost.
app/​models/​lexical_query_builder.rb	Lowers the fulltext field boost.

🧠 Review effort: Lite

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