feat: mempool: don't add pending txs to priority reservoir

[arajasek]

We don't know whether pending txs will actually ever land, so they can throw off our metrics for utilization and priority reservoir.

[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	Dec 22, 2025, 4:53 PM

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 43.73%. Comparing base (cfa7ea6) to head (38b3891).

Files with missing lines	Patch %	Lines
sei-tendermint/internal/mempool/mempool.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2641      +/-   ##
==========================================
+ Coverage   43.71%   43.73%   +0.01%     
==========================================
  Files        1902     1902              
  Lines      158677   158677              
==========================================
+ Hits        69373    69393      +20     
+ Misses      82914    82897      -17     
+ Partials     6390     6387       -3     

Flag	Coverage Δ
sei-chain	45.70% <ø> (+0.01%)	⬆️
sei-cosmos	38.20% <ø> (+<0.01%)	⬆️
sei-db	69.06% <ø> (ø)
sei-tendermint	47.32% <50.00%> (+0.02%)	⬆️

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

Files with missing lines	Coverage Δ
sei-tendermint/internal/mempool/mempool.go	67.78% <50.00%> (ø)

... and 18 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.