Search backend: send multiline matches for unindexed search

Created by: camdencheek

This builds on https://github.com/sourcegraph/sourcegraph/pull/35963 to return multiline matches instead of LineMatch from searcher for all results, not just structural search results. See inline comments for notes.

Stacked on #35963

Test plan

Running backend integration tests, manually tested, and updated all tests, carefully checking correctness.