fix: use createdAt parameter instead of time.Now() in NewProposal

[wen-coding]

Summary

• NewProposal accepted a createdAt time.Time parameter but ignored it, always using time.Now().
• Fix: use the createdAt parameter as intended so callers control the timestamp.
• All existing callers pass time.Now(), so runtime behavior is unchanged.

Test plan

• All existing autobahn tests pass (no behavioral change).

Made with Cursor

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Mar 6, 2026, 2:49 PM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.25%. Comparing base (b866a23) to head (90c3c19).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3030      +/-   ##
==========================================
- Coverage   58.27%   58.25%   -0.02%     
==========================================
  Files        2077     2077              
  Lines      171308   171292      -16     
==========================================
- Hits        99828    99785      -43     
- Misses      62583    62608      +25     
- Partials     8897     8899       +2     

Flag	Coverage Δ
sei-chain-pr	77.94% <100.00%> (?)
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
sei-tendermint/internal/autobahn/types/proposal.go	85.01% <100.00%> (ø)

... and 31 files with indirect coverage changes

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