### What problem does this PR solve?
Fixes Infinity-specific API regressions: preserves ```important_kwd```
round‑trip for ```[""]```, restores required highlight key in retrieval
responses, and enforces Infinity guards for unsupported
```parser_id=tag``` and pagerank in ```/v1/kb/update```. Also removes a
slow/buggy pandas row-wise apply that was throwing ```ValueError``` and
causing flakiness.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
4f036a881d