fix(db2): use DEFAULT for auto-increment columns in inserts - #18422

Pull Request Checklist

• Have you added new tests to prevent regressions?
• If a documentation update is necessary, have you opened a PR to the documentation repository?
• Did you update the typescript typings accordingly (if applicable)?
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Description of Changes

On Db2, Model.create({}) failed when the only value to insert was the auto-increment primary key, e.g. a model without timestamps whose other columns are nullable:

SELECT * FROM FINAL TABLE (INSERT INTO "Events");
-- SQL0104N  An unexpected token "FROM FINAL TABLE (INSERT INTO" was found following "SELECT * "

Db2 disabled supports.autoIncrement.defaultValue, so core dropped the null auto-increment column, and Db2 is the only dialect that supports neither DEFAULT VALUES nor VALUES (), which left a bare INSERT INTO.

bulkCreate([{}, {}]) had its own problems. The Db2 bulkInsertQuery override inserted ids from a counter kept on the query generator (INSERT INTO "Events" VALUES (1),(2)):

• those ids bypass the identity sequence, so later inserts can collide with them;
• it failed with SQL0117N when the table had other columns;
• with returning: true it returned null ids.

Db2 accepts DEFAULT for identity columns, also in multi-row inserts and inside FINAL TABLE. DEFAULT VALUES, VALUES () and a bare VALUES (DEFAULT) on a table with several columns are rejected. So this PR:

• stops disabling supports.autoIncrement.defaultValue and enables supports.bulkDefault for Db2, so inserts use ("id") VALUES (DEFAULT) like PostgreSQL;
• replaces Db2's bulkInsertQuery with the shared implementation, wrapped in SELECT * FROM FINAL TABLE (...) when returning is set, as the IBM i dialect already does. Bulk values are now also escaped with their attribute type.

Replacing the override also means Db2 will get bulk-insert bind parameters from the shared implementation once #17752 lands, which covers the Db2 part of #18346. The two PRs touch the same Db2 method, so whichever merges second needs a small rebase.

Not changed: a Db2 model without any primary key still can't create({}), because there is no column to set to DEFAULT.

Tests

• Integration, for all dialects: create({}) and bulkCreate([{}, {}]) on a model whose values all come from defaults. On main they fail on Db2 with SQL0104N and SQL0117N.
• The Db2-only Worker.create({ id: 1 }) workarounds in belongs-to-many.test.js are removed, and Db2 now shares the PostgreSQL SQL expectation in bulk-create.test.js.
• Unit: per-dialect SQL for an auto-increment column set to null in insertQuery, and for default-only rows in bulkInsertQuery with and without returning. The old Db2 expectation VALUES (0),(NULL) in sql/insert.test.js is now VALUES (0),(DEFAULT); the NULL version fails on the server with SQL0407N.
• model/create, model/bulk-create and associations/belongs-to-many pass on db2, postgres, mysql, mariadb, mssql, sqlite3 and oracle; the Db2 and IBM i unit suites pass.

List of Breaking Changes

None.

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