fix(db2): Include identity offered a clause Db2 does not have

[huyplb]

Tested the identity-insert capability matrix against the live engines instead
of reading it. For each dialect: create a table with an identity column, write an
explicit value using exactly the ceremony the app prescribes, then ask the engine
to generate the next id and see whether it collides.

The bug

Db2 was listed as overriding, and the data-copy path lets overriding through.
Verified on Db2 LUW 11.5 that this can never work:

Attempt	Result
INSERT … (ID, NOTE) OVERRIDING SYSTEM VALUE VALUES …	SQL0104N
clause before the column list	SQL0104N
on INSERT … SELECT	SQL0104N
plain insert into GENERATED ALWAYS	SQL0798N

There is no override clause on Db2 LUW — that is Postgres (and Db2 for i/z-OS).
So "Include identity" started the copy and failed it mid-run, which is the one
outcome this table exists to prevent. Db2 is unsupported now, with the advice
Oracle already gives: switch the toggle off, or make the column GENERATED BY
DEFAULT — which does accept an explicit value, also verified.

The unit tests asserted the same wrong thing as the code, which is exactly why it
survived; they now encode what the servers answered.

What the run confirmed as already correct

• Postgres — OVERRIDING SYSTEM VALUE accepted, and resyncSequence: true
  is right: after writing id 500 the generator is still behind it, so the next
  generated id collides. That is the quiet failure mode, and the matrix has it.
• MySQL — plain insert; the counter moves past the written value
  (resyncSequence: false).
• Oracle — refuses GENERATED ALWAYS with ORA-32795, as documented.
• SQL Server — the session toggle only holds within a single batch; run as
  separate statements it lands on another session and the insert is rejected. The
  migrate path already refuses toggle with a message saying it does not issue
  SET IDENTITY_INSERT, so that limitation is stated honestly.
• GENERATED BY DEFAULT relaxes the requirement on Postgres, Db2 and Oracle.

Verification

npx vitest run — 1799 passed, 95 skipped · tsc --noEmit clean · eslint clean.

🤖 Generated with Claude Code

---

Note

Medium Risk
Changes data-migrate and insert SQL shaping for Db2 identity columns; wrong classification could block valid copies or allow failing runs, but behavior is aligned with live Db2 LUW testing and BY DEFAULT remains supported.

Overview
Db2 LUW no longer uses Postgres-style OVERRIDING SYSTEM VALUE for “Include identity.” The dialect capability table now marks Db2 as unsupported for GENERATED ALWAYS identity writes (with user-facing guidance to turn off Include identity or use GENERATED BY DEFAULT), because live verification on Db2 LUW 11.5 shows the override clause is rejected (SQL0104N) and plain inserts into ALWAYS fail (SQL0798N).

Postgres-family behavior is unchanged — only postgres, cockroachdb, and yugabytedb still get the overriding clause on ALWAYS; Db2 is removed from that list in tests and in buildPeekInsert commentary.

Tests now match engine behavior: Db2 ALWAYS is expected to be refused via identityInsertFor, while Db2 BY DEFAULT remains writable (native).

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