Fix: Infinity keyword round-trip, highlight fallback, and KB update guards (#12660)

### 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)

common/doc_store/infinity_conn_base.py

@@ -367,7 +367,10 @@ class InfinityConnectionBase(DocStoreConnection):
         num_rows = len(res)
         column_id = res["id"]
         if field_name not in res:
-            return {}
+            if field_name == "content_with_weight" and "content" in res:
+                field_name = "content"
+            else:
+                return {}
         for i in range(num_rows):
             id = column_id[i]
             txt = res[field_name][i]