Improve constructorWithExplicitDialect test to verify dialect 1 numeric-as-double behavior

In SQL dialect 1, NUMERIC(18,2) columns are described as DOUBLE PRECISION
instead of scaled integers. The updated test inserts a value into a NUMERIC(18,2)
table column, then selects it using s dialect-1-prepared statement and verifies
that getDouble() retrieves the correct value, exercising the dialect-specific
type mapping.

Author: fdcastel

src/test/Statement.cpp

@@ -175,16 +175,33 @@ BOOST_AUTO_TEST_CASE(constructorWithExplicitDialect)
 {
 	const auto database = getTempFile("Statement-constructorWithExplicitDialect.fdb");
 
-	Attachment attachment{CLIENT, database, AttachmentOptions().setCreateDatabase(true)};
+	// Create a dialect-1 database so that NUMERIC(18, 2) maps to DOUBLE PRECISION.
+	const std::vector<std::uint8_t> dialect1Dpb = {
+		isc_dpb_version1,
+		isc_dpb_sql_dialect,
+		4,
+		1,
+		0,
+		0,
+		0,
+	};
+
+	Attachment attachment{CLIENT, database, AttachmentOptions().setCreateDatabase(true).setDpb(dialect1Dpb)};
 	FbDropDatabase attachmentDrop{attachment};
 
 	Transaction transaction{attachment};
-	Statement stmt{
-		attachment, transaction, "select 1 from rdb$database", StatementOptions().setDialect(SQL_DIALECT_CURRENT)};
+	// In SQL dialect 1, NUMERIC(18,2) is mapped to DOUBLE PRECISION instead of
+	// a scaled integer, so the output descriptor reports DOUBLE and getDouble()
+	// retrieves the value correctly.
+	Statement stmt{attachment, transaction, "select cast(1234.56 as numeric(18, 2)) from rdb$database",
+		StatementOptions().setDialect(1u)};
 
 	BOOST_CHECK(stmt.isValid());
-	BOOST_CHECK(stmt.execute(transaction));
-	BOOST_CHECK_EQUAL(stmt.getInt32(0).value(), 1);
+	BOOST_CHECK(stmt.getOutputDescriptors()[0].adjustedType == DescriptorAdjustedType::DOUBLE);
+	BOOST_REQUIRE(stmt.execute(transaction));
+	auto value = stmt.getDouble(0);
+	BOOST_REQUIRE(value.has_value());
+	BOOST_CHECK_CLOSE(*value, 1234.56, 0.001);
 }
 
 BOOST_AUTO_TEST_SUITE_END()