Add LRU eviction to UberRateLimiter

[Suhaibinator]

Summary

• implement size limited LRU cache in UberRateLimiter
• expose NewUberRateLimiterWithMax
• update rate limiter docs with new memory info
• add README note about limiter cache
• test limiter eviction and adapt existing tests

Testing

• go vet ./...
• go test ./...

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https://chatgpt.com/codex/tasks/task_e_684387b16dfc8331ac352b0b37ac823b

[codecov]

Codecov Report

Attention: Patch coverage is 89.28571% with 3 lines in your changes missing coverage. Please review.

Project coverage is 97.07%. Comparing base (878a259) to head (4f0aba1).

Files with missing lines	Patch %	Lines
pkg/middleware/ratelimit.go	89.28%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #84      +/-   ##
==========================================
- Coverage   97.18%   97.07%   -0.11%     
==========================================
  Files          18       18              
  Lines        2378     2394      +16     
==========================================
+ Hits         2311     2324      +13     
- Misses         55       57       +2     
- Partials       12       13       +1     

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