fix(keystore): flush file before return

[iamquang95]

https://docs.rs/tokio/latest/tokio/fs/struct.File.html

A file will not be closed immediately when it goes out of scope if there are any IO operations that have not yet completed. To ensure that a file is closed immediately when it is dropped, you should call flush before dropping it. Note that this does not ensure that the file has been fully written to disk; the operating system might keep the changes around in an in-memory buffer. See the sync_all method for telling the OS to write the data to disk.

[emlautarom1]

Are there any other similar scenarios (tests or production code) where we should flush? Let's extend the PR with those too if any.

[iamquang95]

We need to flush for tokio::fs::File::write_all only, the tokio::fs::File::write in tokio wraps a blocking call std:: fs::write so it's safe, no need to manually flush. I checked the codebase, and there is only one place use the tokio::fs::File::write_all is in the keystore

[emlautarom1]

Good opportunity to expand our agent skills and include this gotcha https://github.com/NethermindEth/pluto/blob/main/.claude/skills/rust-style/SKILL.md