fix: tolerate Index already exists in Redis Cluster FT.CREATE

[guandeng]

FT._LIST may not return all indices in cluster mode, causing FT.CREATE to fail when the index already exists on another node. Ignore "Index already exists" errors instead of fataling.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.32%. Comparing base (8ff190e) to head (f4ba3e7).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
storage/cache/redis.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1265      +/-   ##
==========================================
+ Coverage   72.96%   73.32%   +0.35%     
==========================================
  Files          91       91              
  Lines       16721    16743      +22     
==========================================
+ Hits        12200    12276      +76     
+ Misses       3276     3252      -24     
+ Partials     1245     1215      -30     

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