perf(batch-prover): replace blocking fs::write with async tokio::fs::write

[anim001k]

Description

Replace blocking std::fs::write with non-blocking tokio::fs::write in the batch prover RPC module to prevent blocking the Tokio worker thread during file I/O operations.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.8%. Comparing base (3593a8a) to head (a7d12b5).
⚠️ Report is 1 commits behind head on nightly.

Files with missing lines	Patch %	Lines
crates/batch-prover/src/rpc.rs	0.0%	4 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/batch-prover/src/rpc.rs	91.9% <0.0%> (-0.9%)	⬇️

... and 5 files with indirect coverage changes

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[anim001k]

@eyusufatik is it ok to be merged?

[kpp]

Please resolve conflicts

[anim001k]

Please resolve conflicts

@kpp resolved

[jfldde]

LGTM but for the nit

[anim001k]

@eyusufatik