Add PrefixSearch for full-text prefix matching (GH-1327)

[jeremydmiller]

Summary

• Fixes Text search on document not returning any results, when desired value is an enum and required parameter is not exact match(Case Insensitive) #1327 — full-text search on enum values stored as strings only matched exact words, not partial prefixes (e.g., "Priced" would not match "PricedIdeaScreening")
• Adds a new PrefixSearch LINQ extension method that uses PostgreSQL's to_tsquery with the :* prefix matching operator, so each search word matches any lexeme that starts with it
• Adds PrefixSearchAsync convenience method on IQuerySession for consistency with the other search methods

How it works

PostgreSQL's to_tsquery('english', 'Priced:*') matches any lexeme starting with "priced" (after stemming). Since "PricedIdeaScreening" is stored as a single lexeme 'pricedideascreening', the prefix 'price:*' matches it.

The PrefixSearch parser transforms the user's input by appending :* to each word and joining with & (AND), then passes it to to_tsquery. For example:

• "Priced" → to_tsquery('english', 'Priced:*')
• "Priced Idea" → to_tsquery('english', 'Priced:* & Idea:*')

Test plan

• New test prefix_search_matches_partial_enum_value_GH_1327 verifies that PrefixSearch("Priced") matches a document containing "PricedIdeaScreening" while standard Search("Priced") does not
• Existing NgramSearch tests pass (7/7)
• Build succeeds with 0 errors

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