feat(sql): construct Postgres and Oracle providers; MySql/Odbc stay reserved

Widens the two provider gates so an authored `{"provider":"Postgres"}` or
`{"provider":"Oracle"}` binds that provider's own dialect instead of failing
"not available in this build":

- `SqlDriverFactoryExtensions.CreateDialect` gains a Postgres and an Oracle
  arm. There is deliberately no fallback arm — a default resolving to
  SqlServerDialect would construct happily and then emit T-SQL at a Postgres
  or Oracle server. The rejection message now names the three supported and
  the two still-reserved providers.
- `SqlDriverBrowser.DefaultDialectSelector` widens identically, so a provider
  the runtime constructs is also browsable; MySql/Odbc keep resolving to null
  and the caller keeps reporting them as unavailable.

Tests: the existing rejected-provider controls flip from Postgres to MySql
(factory) and Odbc (browser) so the reserved half stays falsifiable, and the
new legs assert the bound dialect TYPE rather than mere construction —
"no throw" alone would pass against a SqlServer fallback.

Doc-comment sweep (contract addition, one line each): the "v1 constructs
SqlServer only" / "reserved for P2" claims in SqlProvider, SqlDriverConfigDto,
ISqlDialect.Provider and SqlServerDialect are now false and were corrected in
the same change.

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

@@ -201,17 +201,27 @@ public static class SqlDriverFactoryExtensions
 
     /// <summary>
     ///     Selects the provider seam.
-    ///     <para>v1 constructs <see cref="SqlProvider.SqlServer"/> only. The other members exist on the shipped
-    ///     enum as reserved names; authoring one is a config mistake that must fail at construction with a
-    ///     message that says so, rather than silently degrading to T-SQL against a non-SQL-Server database.</para>
+    ///     <para>This build constructs <see cref="SqlProvider.SqlServer"/>, <see cref="SqlProvider.Postgres"/>
+    ///     and <see cref="SqlProvider.Oracle"/>. <see cref="SqlProvider.MySql"/> and
+    ///     <see cref="SqlProvider.Odbc"/> remain reserved names on the shipped enum; authoring one is a config
+    ///     mistake that must fail at construction with a message that says so, rather than silently degrading
+    ///     to another backend's SQL dialect against a database that does not speak it.</para>
+    ///     <para><b>Every arm returns that provider's OWN dialect — there is no fallback arm.</b> A default
+    ///     that resolved to <see cref="SqlServerDialect"/> would construct happily and then emit T-SQL
+    ///     (<c>INFORMATION_SCHEMA</c>, <c>TOP 1</c>, <c>@</c> markers) at a Postgres or Oracle server, which
+    ///     fails at poll time as an unintelligible provider error rather than here as a config error.</para>
     /// </summary>
     private static ISqlDialect CreateDialect(SqlProvider provider, string driverInstanceId)
         => provider switch
         {
             SqlProvider.SqlServer => new SqlServerDialect(),
+            SqlProvider.Postgres => new PostgresDialect(),
+            SqlProvider.Oracle => new OracleDialect(),
             _ => throw new InvalidOperationException(
                 $"Sql driver config for '{driverInstanceId}' names provider '{provider}', which is not " +
-                $"available in this build. Only '{nameof(SqlProvider.SqlServer)}' is supported."),
+                $"available in this build. Supported: '{nameof(SqlProvider.SqlServer)}', " +
+                $"'{nameof(SqlProvider.Postgres)}', '{nameof(SqlProvider.Oracle)}'. Reserved (declared on " +
+                $"the enum but not implemented): '{nameof(SqlProvider.MySql)}', '{nameof(SqlProvider.Odbc)}'."),
         };
 
     /// <summary>