refactor(insert): allowlist DEFAULT keyword to pg; other dialects throw

Invert the heterogeneous-rows branch: instead of special-casing
sqlite to throw and assuming every other dialect accepts the DEFAULT
keyword, only postgres (known to support it) gets DEFAULT and any
other dialect raises. A future dialect fails loud on first use
instead of emitting SQL that may parse as something else.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: ryanrasti

src/builder/insert.ts

@@ -48,18 +48,20 @@ export class FinalizedInsert<Name extends string, T extends TableBase, R extends
       const vals = usedColumns.map((k) => {
         const v = row[k];
         if (v === undefined) {
-          // Some other row provides this column, this one doesn't. PG
-          // spells that `DEFAULT`; SQLite has no per-row spelling for
-          // it, and silently inserting NULL would diverge from what
-          // omitting the column means (rowid auto-fill, declared
-          // DEFAULT). Make the caller decide.
-          if (tableCls.database.dialect === "sqlite") {
-            throw new Error(
-              `Insert into '${tableName}': column '${k}' is set in some rows but not others. ` +
-              `SQLite cannot express "use the column default" per row — provide '${k}' in every row or in none.`,
-            );
+          // Some other row provides this column, this one doesn't.
+          // Only PG is known to spell that `DEFAULT`; for any other
+          // dialect (SQLite has no per-row spelling for it), silently
+          // inserting NULL would diverge from what omitting the column
+          // means (rowid auto-fill, declared DEFAULT) — make the
+          // caller decide.
+          if (tableCls.database.dialect === "postgres") {
+            return sql`DEFAULT`;
           }
-          return sql`DEFAULT`;
+          throw new Error(
+            `Insert into '${tableName}': column '${k}' is set in some rows but not others. ` +
+            `The '${tableCls.database.dialect}' dialect cannot express "use the column default" per row — ` +
+            `provide '${k}' in every row or in none.`,
+          );
         }
         const col = getColumn(instance, k);
         return col[meta].__class.from(v).toSql();