fix: make connection optional when literal connection string is configured

When SqlCommandProvider or SqlProgrammabilityProvider is configured with
a literal connection string at design time, the connection parameter in
the generated constructor/factory method was incorrectly required. This
meant users had to pass an open SqlConnection even when a literal
connection string was available.

The fix makes the connection parameter always optional (defaulting to
null). The existing body2 logic already handles null by falling back to
the design-time connection string expression, so no further changes
are needed.

Users can now call:
  AdventureWorks.CreateCommand<"SELECT ...">().Execute()
instead of having to first open a SqlConnection.

Closes #245

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

src/SqlClient.DesignTime/DesignTime.fs

@@ -652,7 +652,7 @@ type DesignTime private() =
                         ProvidedParameter(
                             "connection", 
                             typeof<SqlConnection>,
-                            ?optionalValue = if designTimeConnectionString.IsDefinedByLiteral then None else Some null
+                            ?optionalValue = Some null
                         )           
                         ProvidedParameter("transaction", typeof<SqlTransaction>, optionalValue = null) 
                         ProvidedParameter("commandTimeout", typeof<int>, optionalValue = SqlCommand.DefaultTimeout)