fix(durable-sqlite): surface migration error via Error.cause on rollback

[spokodev]

Fixes #5763.

In the durable-sqlite migrator, tx.rollback() throws a sentinel TransactionRollbackError to abort the surrounding transaction. The previous code looked like:

```ts
} catch (error: any) {
tx.rollback(); // throws TransactionRollbackError
throw error; // never reached
}
```

Because `tx.rollback()` throws synchronously, the `throw error` below it never executes. The caller only ever saw the generic `"Rollback"` message — the real cause (failed SQL statement, schema mismatch, etc.) was discarded. This matched the symptom @Craig-J described in #5763.

The fix wraps the rollback in a try/catch and attaches the original migration error as `rollbackError.cause` before re-throwing, so callers can recover the underlying failure:

```ts
} catch (error: any) {
try {
tx.rollback();
} catch (rollbackError: any) {
rollbackError.cause = error;
throw rollbackError;
}
throw error;
}
```

Behaviour change

Standalone verification (mocking the tx interface):

```
--- before ---
caller sees: TransactionRollbackError / Rollback
caller sees .cause: undefined

--- after ---
caller sees: TransactionRollbackError / Rollback
caller sees .cause: Migration 0003 failed: column "x" does not exist
```

Test plan

• `pnpm test:types` clean
• Manual repro via a standalone script reproducing the mocked transaction/rollback flow (script not committed; behaviour above)
• No durable-sqlite test infra exists in the repo; a Workers-runtime test would require miniflare and is out of scope for this fix