fix(parser): preserve T-SQL OFFSET/FETCH across parse→generate roundtrip (CR-010)

[yigalrozenberg]

Summary

Fixes CR-010 / PSQ-2420: T-SQL FETCH NEXT m ROWS ONLY was silently dropped during parse → generate roundtrips, causing data over-exposure in the Secure Query Gateway whenever an ABAC policy was active on a LIMIT/OFFSET query.

Root cause

The defect is in the parser, not the generator. After parsing OFFSET <expr>, the parser did not consume the trailing ROWS/ROW keyword. So for input like:

```
ORDER BY col OFFSET 2 ROWS FETCH NEXT 5 ROWS ONLY
```

the next `match_token(Fetch)` saw `ROWS` and returned `false`, leaving `fetch_first = None`. The generator already emits `OFFSET n ROWS FETCH NEXT m ROWS ONLY` correctly for T-SQL/Fabric when `fetch_first` is set, which is why direct transpile worked but re-parsing the transpiled SQL did not.

Fix

Two-line change in `src/parser/sql_parser.rs`: after parsing the OFFSET expression, consume the optional `ROWS`/`ROW` keyword (ANSI SQL:2008 / T-SQL form).

Tests

Added 4 tests in `tests/test_dialects.rs`:

• `test_tsql_offset_fetch_roundtrip_preserved`
• `test_tsql_offset_fetch_with_subquery_order_roundtrip`
• `test_tsql_offset_only_no_fetch_roundtrip` (regression guard — OFFSET without FETCH must NOT add FETCH)
• `test_pg_limit_offset_transpile_then_tsql_roundtrip` (end-to-end ABAC pipeline scenario)

All previously passing tests continue to pass (zero regressions across the full suite).

Validation

• `cargo test` — all suites green
• `cargo fmt --all` clean
• `cargo clippy` — no new warnings on changed lines

Refs: PSQ-2420