…head/rewind (#21570)

Fix some idempotency tests that failed in https://github.com/oxc-project/monitor-oxc/actions/runs/24652862465/job/72079552802

## Summary

When `/* @__PURE__ */` appears before an object literal (e.g. `export const X = /* @__PURE__ */ { a: 1 }`), the parser's arrow-function lookahead causes the comment to be added to the `comments` vec during speculative parsing. After rewind, the same comment is encountered again — the dedup check skips the push, but `parse_annotation` had already set `pure_comment` to `comments.len()`, which now points to a non-existent index. This causes `mark_pure_comment_not_applied` to silently fail, breaking codegen idempotency.

### Fix

- Move `pure_comment` / `has_no_side_effects_comment` assignment from `parse_annotation` to `add_comment`, **after** the dedup check
- In the dedup branch, set the index to the existing comment (`len - 1`)
- In the normal branch, set the index to where the comment will be pushed (`len`)
- Extract shared logic into `set_annotation_flags` helper

## Test plan

- Added parser test: `/* @__PURE__ */` before object literal is correctly marked `PureNotApplied`
- Added parser test: `/* @__PURE__ */` before call expression remains `Pure` after lookahead
- Added codegen idempotency test: parse→codegen→parse→codegen produces identical output
- `cargo test -p oxc_parser --lib -- trivia_builder` — 9 tests pass
- `cargo test -p oxc_codegen --test integration -- comments` — all pass