feat(giga): fallback to v2 for associations and migrations

[arajasek]

Describe your changes and provide context

#2994 introduced an iterator for the cachekv store for the specific purpose of migrating balances during association. There are concerns with that change -- in particular future stores won't support iterators at all.

Instead, we can simply fallback to v2 when associatons need to occur, similar to how we handle interop. This PR makes that change, and adds a test for it.

Testing performed to validate your change

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Mar 24, 2026, 9:03 PM

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.75%. Comparing base (7fd79a0) to head (439daa1).

Files with missing lines	Patch %	Lines
giga/executor/precompiles/failfast.go	50.00%	2 Missing ⚠️
giga/deps/store/cachekv.go	0.00%	1 Missing ⚠️
giga/deps/xbank/keeper/view.go	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3103      +/-   ##
==========================================
+ Coverage   58.73%   58.75%   +0.01%     
==========================================
  Files        2094     2094              
  Lines      173026   172978      -48     
==========================================
+ Hits       101627   101631       +4     
+ Misses      62419    62370      -49     
+ Partials     8980     8977       -3     

Flag	Coverage Δ
sei-chain-pr	57.00% <66.66%> (?)
sei-db	70.41% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
app/app.go	68.83% <100.00%> (+1.39%)	⬆️
giga/deps/store/cachekv.go	55.55% <0.00%> (+13.53%)	⬆️
giga/deps/xbank/keeper/view.go	91.11% <80.00%> (-1.62%)	⬇️
giga/executor/precompiles/failfast.go	33.33% <50.00%> (+6.06%)	⬆️

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