fix(drizzle-kit): validate journal entries before converting migrations folder - #6170

Summary

Fixes #6166.

migrateToFoldersV3() (the drizzle-kit up v0→v3 folder conversion) processed journal entries incrementally and destructively: for each entry it created the new folder, wrote snapshot.json + migration.sql, then deleted the original flat .sql. A journal entry whose snapshot file is missing was only discovered when the loop reached it — by which point every earlier entry had already been converted and its original deleted. Result: a half-converted migrations folder that drizzle-kit up can never finish (re-running hits MigrationsOutdatedCliError), with an error message ("No snapshot was found") that didn't even name the missing file.

Changes:

1. Validate every entry up-front before writing anything: each entry's snapshot must exist, and any SQL-file ambiguity (MigrationSqlFilesConflictCliError) is detected in the same pre-pass. If anything is wrong, nothing is converted — originals stay intact and the folder remains resumable once fixed.
2. Name the missing path in the error: No snapshot was found for '<path>' so users know exactly which journal entry is broken.

Testing

New regression tests in drizzle-kit/tests/other/migrate-folders-v3.test.ts (pure file-based, no DB):

• a two-entry journal where the second snapshot is missing → throws, and asserts nothing was converted: both original .sql files intact, meta/ untouched, no new folders created
• a complete folder still converts fully (meta removed, originals moved)

Red/green verified: without the fix the atomicity test fails (entry 0 already converted + original deleted when the error throws); with the fix both pass.

✓ tests/other/migrate-folders-v3.test.ts (2 tests) 14ms

 Test Files  1 passed (1)
      Tests  2 passed (2)