translate/upsert: use emit_column_or_rowid for DO UPDATE row snapshot

[ongyimeng]

Description

• In core/translate/upsert.rs, conflicting-row snapshots now use emit_column_or_rowid(...)
• Replaces manual Column { default: None } handling
• Adds regression test in testing/sqltests/tests/upsert.sqltest

Motivation and context

• UPSERT was reading NULL in cases where SQLite expects the column’s default value
• This happens for missing trailing column values after ALTER TABLE ... ADD COLUMN ... DEFAULT ...
• The fix restores SQLite-compatible behavior
• Applies specifically to the DO UPDATE path of UPSERT

Issue #6087

Description of AI Usage

Used AI to trace the issue to upsert.rs, propose the minimal fix, and draft the regression test.

[turso-bot]

Please review @LeMikaelF

[turso-bot]

Please review @LeMikaelF

[ongyimeng]

could someone please help rerun the failed CI jobs for this PR? I believe the failures are infra/flaky