review(Driver.Sql): findings + fix the missing default-schema fallback, the rowSelector doc, and a dead package ref

Driver.Sql-001 (Medium): SqlCatalogLoader.ReadDefaultSchemaAsync documented a
fallback to the sole visible schema when the server reports no default schema and
did not implement it, so a login with no DEFAULT_SCHEMA (or a Postgres search_path
that resolves to nothing) rejected EVERY unqualified authored table at the catalog
gate — a whole-driver BadNodeIdUnknown outage against a healthy database. Fixed;
SqlCatalogDefaultSchemaTests written red first, with a control proving the
ambiguous multi-schema case still refuses to guess.

Driver.Sql-003 (Medium): docs/drivers/Sql.md spelled the WideRow row selector at
the top level, but the parser reads those keys only inside a nested rowSelector —
a tag authored from the doc is refused and reads BadNodeIdUnknown (the #552 class,
via the documentation). Also removed the documented-but-nonexistent pollIntervalMs
per-tag field.

Driver.Sql-007 (Low): dropped the unused Microsoft.Data.SqlClient PackageReference
left behind by the .Dialect extraction (#526).

Open: 002 (no IN-list chunking vs the 2100/1000 provider bind caps), 004, 005, 006,
008, 009, 010.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Sql/SqlCatalogLoader.cs

@@ -74,7 +74,7 @@ internal static class SqlCatalogLoader
                 "the catalog views, which is a grant problem rather than a tag-authoring one");
 
         var defaultSchema = await ReadDefaultSchemaAsync(
-            connection, dialect, timeoutSeconds, cancellationToken).ConfigureAwait(false);
+            connection, dialect, schemas, timeoutSeconds, cancellationToken).ConfigureAwait(false);
 
         var tablesBySchema = new Dictionary<string, IReadOnlyList<string>>(StringComparer.Ordinal);
         var columnsByTable = new Dictionary<string, IReadOnlyList<string>>(StringComparer.Ordinal);
@@ -132,12 +132,23 @@ internal static class SqlCatalogLoader
     ///     query answers null/blank.
     /// </summary>
     /// <remarks>
-    ///     A null answer is possible (a login with no default schema mapped). Falling back to the single
-    ///     visible schema keeps the common one-schema database working; with several visible schemas there is
-    ///     no defensible guess, so unqualified names simply will not resolve and the gate says so by name.
+    ///     <para>A null answer is possible (a login with no default schema mapped, or a Postgres connection
+    ///     whose <c>search_path</c> resolves to nothing). Falling back to the single visible schema keeps the
+    ///     common one-schema database working; with several visible schemas there is no defensible guess, so
+    ///     unqualified names simply will not resolve and the gate says so by name.</para>
+    ///     <para><b>The fallback is load-bearing, not cosmetic.</b> Without it a blank answer resolves the
+    ///     default schema to the empty string, the tables query runs for schema <c>''</c>, it matches nothing,
+    ///     and <em>every unqualified tag on the driver</em> is rejected by the gate and reads
+    ///     <c>BadNodeIdUnknown</c> — a whole-driver outage against a healthy database with every tag correctly
+    ///     authored. Guessing among several schemas is the opposite error (a tag silently bound to the wrong
+    ///     relation), which is why the fallback stops at exactly one candidate.</para>
     /// </remarks>
     private static async Task<string> ReadDefaultSchemaAsync(
-        DbConnection connection, ISqlDialect dialect, int timeoutSeconds, CancellationToken cancellationToken)
+        DbConnection connection,
+        ISqlDialect dialect,
+        IReadOnlyList<string> schemas,
+        int timeoutSeconds,
+        CancellationToken cancellationToken)
     {
         var command = connection.CreateCommand();
         await using (command.ConfigureAwait(false))
@@ -147,7 +158,11 @@ internal static class SqlCatalogLoader
             command.CommandTimeout = timeoutSeconds;
             var value = await command.ExecuteScalarAsync(cancellationToken).ConfigureAwait(false);
             var schema = value is null or DBNull ? null : value.ToString();
-            return string.IsNullOrWhiteSpace(schema) ? string.Empty : schema;
+            if (!string.IsNullOrWhiteSpace(schema)) return schema;
+
+            // No default schema for this login: the sole visible schema is the only defensible answer, and
+            // with more than one there is none — the empty string then resolves nothing, by design.
+            return schemas.Count == 1 ? schemas[0] : string.Empty;
         }
     }
 

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Sql/ZB.MOM.WW.OtOpcUa.Driver.Sql.csproj

@@ -18,7 +18,11 @@
     <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Driver.Sql.Dialect\ZB.MOM.WW.OtOpcUa.Driver.Sql.Dialect.csproj"/>
   </ItemGroup>
   <ItemGroup>
-    <PackageReference Include="Microsoft.Data.SqlClient" />
+    <!-- No provider package here on purpose: every type in this project speaks System.Data.Common
+         only (DbProviderFactory / DbConnection / DbCommand), and the concrete provider packages live
+         behind the .Dialect leaf referenced above. A direct Microsoft.Data.SqlClient reference sat
+         here unused after that extraction and read as "this project is SQL-Server-specific", which
+         is exactly the assumption the dialect seam exists to prevent. -->
     <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" />
   </ItemGroup>
   <ItemGroup>