Add OrderByNgramRank for relevance-sorted ngram search results

[jeremydmiller]

Summary

• New LINQ extension: OrderByNgramRank(x => x.Field, "search term")
• Generates ORDER BY ts_rank(mt_grams_vector(...), mt_grams_query(...)) DESC
• Allows ngram search results to be sorted by relevance without raw SQL
• Works with Select(), Take(), and other LINQ operators

Closes #4037

Usage

var results = await session
    .Query<User>()
    .Where(x => x.SearchString.NgramSearch("search term"))
    .OrderByNgramRank(x => x.SearchString, "search term")
    .ToListAsync();

Implementation

• New OrderByNgramRankOperator LinqOperator that captures the member expression and search term
• Extended Ordering class with NgramRank-specific properties for deferred member resolution at SQL compilation time
• Added internal Options accessor to CollectionUsage for schema name access

Test plan

• 2 new tests (basic ranking, ranking with Select projection)
• 12/12 total ngram tests passing, zero regressions

🤖 Generated with Claude Code