fix(codegen): write prettier-stable schema snapshot

[kitlangton]

What

Regenerating a database migration reformats the entire checked-in packages/core/schema.json, burying the real change in hundreds of lines of array-reflow noise. This applies the same treatment #23228 gave the OpenAPI documents: format the generated snapshot in place so regeneration diffs contain only real schema changes.

Before / After

Before: bun script/migration.ts raw-copies drizzle's snapshot.json, which emits every array multi-line. The checked-in snapshot is prettier-styled (short arrays collapsed), so adding a single column reflows every "columns": [...] array in the file — a 3-column addition produced a 206-line diff, ~170 of them formatting churn.

After: the snapshot is piped through prettier (parser: "json", printWidth: 120) before writing. The formatter is a byte-for-byte no-op on the current checked-in snapshot, and a test column addition now produces a 14-line diff.

How

• packages/core/script/migration.ts: replaces the fs.copyFile of drizzle's snapshot with a formatted Bun.write, adding a formatJson helper mirroring the existing formatTypescript.
• No behavior change for consumers: --check feeds the snapshot to drizzle as a parsed baseline and never byte-compares it, so formatting is invisible to validation.

Testing

• Verified the formatter is a no-op on the checked-in schema.json (byte-identical input/output).
• Exercised the write path with a throwaway column: generated migration, confirmed the new snapshot stays prettier-compact and diffs at 14 lines, then reverted.
• bun script/migration.ts --check and bun typecheck clean from packages/core.