refactor(dedup): share TruncateText across Markdown generators

`TruncateText` was copy-pasted verbatim in three Markdown generators
(`SqlServer.QueryBreakdownGenerator`, `SqlServer.SqlStatementGenerator`,
`Expressions.ExpressionGenerator`). Pulled out to a new
`Strata.SqlTools.Markdown.Internal.MarkdownTextHelpers` static class
(internal — no public-API change).

Each call site keeps its own one-line private wrapper for source
readability so existing `TruncateText(...)` calls in the generators
need no edits.

Deliberately *not* unified across the same three files:
- `EscapeMermaidText` (QueryBreakdownGenerator) vs `EscapeMermaidText`
  (SqlStatementGenerator) — the QBG version intentionally escapes
  `[]{}()` for Mermaid node syntax; the SSG version only escapes
  quotes/newlines because it writes into `Note right of DB: ...`
  contexts where brackets render fine.
- `EscapeMarkdown` (ExpressionGenerator) — a different escape set
  again, targeting Markdown rather than Mermaid.

Also deliberately *not* refactored: `SqlServer.UpdateBreakdown.TryParse`
≡ `SqlServer.ProcedureBreakdown.TryParse` prelude (empty-check +
parser + prefix regex + extract setup/finish clauses). The 30-line
duplication is real, but every extraction shape (tuple return,
`out`-flavored helper, context type) is measurably worse than the
duplicated original. Leaving it.

All 1180 tests stay green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Strata.SqlTools.Markdown/Expressions/ExpressionGenerator.cs

@@ -361,13 +361,7 @@ public class ExpressionGenerator : IVisitor<string>
     }
 
     private static string TruncateText(string text, int maxLength)
-    {
-        if (string.IsNullOrWhiteSpace(text) || text.Length <= maxLength)
-        {
-            return text;
-        }
-        return string.Concat(text.AsSpan(0, maxLength), "...");
-    }
+        => Internal.MarkdownTextHelpers.TruncateText(text, maxLength);
 
     #region IVisitor Implementation