Return proper error message when SS disabled

[yzang2019]

Describe your changes and provide context

Currently if SS is disabled and one tries to query historical state, you'll get error like:

Error: rpc error: code = Unknown desc = kv store with key KVStoreKey{0xc000c88020, acc} has not been registered in stores: panic

However, it is not explicit and we should return the correct error message to indicate this was due to misconfiguration.

Testing performed to validate your change

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Feb 18, 2026, 3:45 AM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.30%. Comparing base (07025b9) to head (1696b53).

❗ There is a different number of reports uploaded between BASE (07025b9) and HEAD (1696b53). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (07025b9)	HEAD (1696b53)
sei-chain	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2911       +/-   ##
===========================================
- Coverage   57.22%   48.30%    -8.92%     
===========================================
  Files        2094      671     -1423     
  Lines      171956    50626   -121330     
===========================================
- Hits        98400    24456    -73944     
+ Misses      64739    23988    -40751     
+ Partials     8817     2182     -6635     

Flag	Coverage Δ
sei-chain	?
sei-cosmos	48.12% <100.00%> (-0.08%)	⬇️
sei-db	68.72% <ø> (?)

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

Files with missing lines	Coverage Δ
sei-cosmos/storev2/rootmulti/store.go	31.87% <100.00%> (-9.98%)	⬇️

... and 1554 files with indirect coverage changes

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