Reject NaN and Inf in float32 vector input

NaN/Inf values in vectors break heap/sort invariants in KNN, causing
wrong or unpredictable results. Now rejected at parse time in
fvec_from_value() for both blob and JSON text input paths, with a
clear error message identifying the offending element index.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>