[Orm] Skip executing fully-commented introspect migration files

[youcefzemmar]

## What

`readMigrationFiles` now recognises the introspection SQL file that `drizzle-kit pull` emits — two `--` notes followed by the entire schema wrapped in a single `/* ... */` block — and returns `sql: []` for that entry instead of feeding the comment delimiters to the driver. The migration is still hashed and inserted into `__drizzle_migrations`, so it's recorded as an applied baseline.

The detection is intentionally narrow: it only kicks in when the body (after stripping leading `--` lines and whitespace) starts with `/*`, ends with `*/`, and has no nested `*/`. Anything else falls through to the existing `--> statement-breakpoint` split.

## Why

Reproduced in #4851. `drizzle-kit pull` writes the schema commented out and asks the user to uncomment before running `migrate`. People (`@joshbodily`, `@melroy89` in the thread) are using the commented file on purpose, as a "this schema is already in the DB, just record it as applied" baseline. Today `drizzle-kit migrate` crashes with a parse error because the runtime tries to execute the `/* ... */` wrapper.

Treating the no-op file as zero statements matches what users were already doing manually (deleting the file or hand-editing the journal) and keeps the journal/hash row in place so subsequent migrations diff cleanly.

## Testing

- Added `drizzle-orm/tests/migrator.test.ts` covering: the commented introspect file → `sql: []`; a normal `--> statement-breakpoint`-split file is unaffected; an uncommented introspect file still executes; a file with SQL after a trailing block comment is not skipped.
- I couldn't run `pnpm test` locally — `pnpm` errors with "unable to open database file" in this sandbox before it can install. Verified the detection logic directly with `node` against the four fixtures and the project's `tsc --noEmit` against the touched files.

## Notes

- `readMigrationFiles` is shared by every dialect migrator (pg, mysql, sqlite, libsql, neon, vercel, etc.); the change is read-only and only fires on files that today crash, so working migrations are unaffected — the "normal multi-statement" test covers that explicitly.
- Detection is conservative on purpose: nested `*/`, mixed real SQL + comment block, or any non-`--`/`/* */` content disables the skip. Risk of silently dropping a real migration is low.
- Bumped `changelogs/drizzle-orm/0.45.4.md` with a one-liner.

Fixes #4851.