fix(store): handle nil end in cachekv/iterator

[kocubinski]

Description

Closes: #12661

Adds support for nil end semantics to iterators in cachekv store, addressing this workaround.

Note that this has the effect of sorting the dirty cache into the BTree cache store in the bounds [startIndex, end-of-cache-space]

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Author Checklist

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I have...

• included the correct type prefix in the PR title
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• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
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• confirmed the correct type prefix in the PR title
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• reviewed state machine logic
• reviewed API design and naming
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[codecov]

Codecov Report

Merging #12945 (2e3f287) into main (b515372) will increase coverage by 0.03%.
The diff coverage is 45.45%.

@@            Coverage Diff             @@
##             main   #12945      +/-   ##
==========================================
+ Coverage   55.46%   55.49%   +0.03%     
==========================================
  Files         646      646              
  Lines       54865    54870       +5     
==========================================
+ Hits        30429    30452      +23     
+ Misses      21984    21963      -21     
- Partials     2452     2455       +3     

Impacted Files	Coverage Δ
store/cachekv/store.go	86.61% <45.45%> (+7.49%)	⬆️

[alexanderbez]

ohhh, changelog entry too pls :)