Fix flaky test: TestParquetFilePruning

[jewei1997]

Describe your changes and provide context

TestParquetFilePruning was flaky because it relied on a background pruning goroutine that fires immediately on store open. The test queried data without synchronizing with the concurrent pruning, creating a race between file deletion and DuckDB queries that failed frequently on slower hardware.

Fix:

• Disable background pruning in the test (PruneIntervalSeconds = 0) and trigger
  PruneOldFiles synchronously instead
• Export PruneOldFiles on parquet.Store (renamed from pruneOldFiles) to enable this
• Add assertions that files were actually removed from disk (before vs after
  counts), making the test more thorough
• Replace common.Big1.SetUint64(block) with new(big.Int).SetUint64(block) to avoid
  mutating a shared global

Testing performed to validate your change

go test ./sei-db/ledger_db/receipt/ -run TestParquetFilePruning -race -count=5
  -timeout 120s
go test ./sei-db/ledger_db/parquet/ -count=1 -timeout 120s
go test ./sei-db/ledger_db/receipt/ -count=1 -timeout 180s

[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 12, 2026, 1:12 PM

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 58.33%. Comparing base (8724492) to head (1cf1aad).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
sei-db/ledger_db/parquet/store.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3053      +/-   ##
==========================================
+ Coverage   58.28%   58.33%   +0.04%     
==========================================
  Files        2072     2079       +7     
  Lines      171414   170913     -501     
==========================================
- Hits        99909    99696     -213     
+ Misses      62584    62278     -306     
- Partials     8921     8939      +18     

Flag	Coverage Δ
sei-chain-pr	73.84% <50.00%> (?)
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-db/ledger_db/parquet/store.go	68.78% <50.00%> (-0.61%)	⬇️

... and 245 files with indirect coverage changes

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[masih]

@jewei1997 resolved conflicts on this one to unblock merging this. Since this test is flaking regularly across builds.