search: Remove `file:contains()`

Created by: tbliu98

Straggler from #39767 (closed)

Removes file:contains() from list of valid predicates, in line with repo:contains() no longer being a valid predicate.

Removes query syntax highlighting for file:contains() and repo:contains() to visually indicate these queries will not be interpreted as predicates.

Updates examples used in comments and docs

Updates metrics collection to track updated predicates

Test plan

Added and updated unit tests Backend integration tests