Add Query Store query-text search filter

[erikdarlingdata]

Summary

• Adds a server-side query-text search to the Query Store integration, so users can find plans touching a specific table or statement fragment even when those plans aren't in the top N by the chosen metric — mirroring sp_QuickieStore's @query_text_search default behavior (requested in [FEATURE] Query text search #389).
• The filter is applied pre-TOP-N in all three Query Store SQL builders (flat, grouped-by-hash, grouped-by-module), fully parameterized, and exposed with parity across all three surfaces: Desktop Search-by combo, MCP get_query_store_top, and the CLI query-store command.
• Semantics match the proc's defaults exactly: input is trimmed and auto-wrapped in % when missing; %, _, [, ] remain live LIKE wildcards (bracket escaping is a possible follow-up, matching @escape_brackets).

Closes #389

Changes

• Core: QueryStoreFilter.QueryTextSearch property + shared QueryStoreFilter.BuildQueryTextSearchPattern helper (QueryStorePlan.cs); identical parameterized EXISTS predicate against sys.query_store_query_text added to FetchTopPlansAsync and both grouped builders (QueryStoreService.cs, QueryStoreService.Grouped.cs). With the filter unset, generated SQL is byte-identical to before.
• Desktop: "Query Text" item in the Search-by combo + one switch case (QueryStoreGridControl.axaml, QueryStoreGridControl.Filters.cs).
• MCP: additive query_text_search parameter on get_query_store_top (named to avoid colliding with the tool's existing query_text output field).
• CLI: additive --query-text-search option.
• Tests: QueryTextSearchPatternTests.cs — full wrap/trim/wildcard/null matrix for the helper, plus backfilled coverage for the previously untested ParseExecutionType (22 new cases; 123 total pass).

Test Plan

• dotnet build PlanViewer.sln -c Debug — no new warnings
• dotnet test — 123/123 pass (101 existing + 22 new)
• Live end-to-end on SQL Server 2022 / StackOverflow2013: unfiltered top-5 vs --query-text-search Votes return different query sets (proving pre-TOP-N filtering); no-match input exits cleanly with "no data"; returned query verified to match via substring (DownVotes)
• planview query-store --help shows the new option with wildcard semantics
• Desktop Search-by "Query Text" smoke (code-verified; UI not driven)

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