Support WITHSCORES/SCORER in cluster search (CSS) mode

Problem:

FT.SEARCH with WITHSCORES/SCORER is currently blocked in cluster search (CSS) mode with an error:

`WITHSCORES/SCORER is not yet supported in cluster search mode`

The coordinator-side reply parser in `FtSearchCSS` expects each hit as `[key, [sortkey], [fields...]]`, but WITHSCORES adds an extra score element `[key, score, [sortkey], [fields...]]` which misaligns parsing and produces errors.

Blocked in #7181.

Required changes:
1. **CSS reply parser**: when `params.with_scores` is set, consume the score string after the key and populate `SerializedSearchDoc::text_score`
2. **Score merging**: when collecting results from multiple shards, scores from different shards use per-shard statistics. Consider whether cross-shard score normalization is needed or if per-shard scores are acceptable for ranking
3. **SCORER forwarding**: ensure the SCORER parameter is forwarded to remote shards in the CSS command string
4. **Remove the guard** added in #7181 once the above is implemented

Start with the coordinator-side FtSearchCSS reply parser and SerializedSearchDoc::text_score, then inspect the CSS command construction and the guard added in #7181. Verify that WITHSCORES parses correctly across shard replies, SCORER reaches remote shards, score merging is handled appropriately, and the unsupported-mode guard is removed.

Written by the indexing model from the issue text.