Resolve type issues in redis cache

[jackgerrits]

Why are these changes needed?

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 46.82%. Comparing base (09a4918) to head (c2c944b).

Files	Patch %	Lines
autogen/cache/redis_cache.py	84.61%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1872       +/-   ##
===========================================
+ Coverage   36.02%   46.82%   +10.80%     
===========================================
  Files          63       63               
  Lines        6621     6627        +6     
  Branches     1462     1588      +126     
===========================================
+ Hits         2385     3103      +718     
+ Misses       4041     3272      -769     
- Partials      195      252       +57     

Flag	Coverage Δ
unittests	46.74% <84.61%> (+10.72%)	⬆️

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[qingyun-wu]

LGTM!