fix(securefile,credstore): call Sync on temp file before close and rename

[euxaristia]

Summary

Fixes a High-severity durability issue where credential and secret files could be corrupted or lost (0-byte file) if a system crash or power failure occurs shortly after a write.

Although a write-to-temp-then-rename pattern is used, neither securefile nor credstore called tmp.Sync() (fsync) to flush the page cache and directory metadata to physical disk before renaming.

This PR adds Sync() calls (and checks their errors) before closing and renaming.

Changes

• In internal/securefile/securefile.go: Call tmp.Sync() in writeNewSecretFile.
• In internal/credstore/credstore.go: Call tmp.Sync() in Store.write.

Test plan

• go test -race ./internal/securefile/... ./internal/credstore/... — ok

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📥 Commits

Reviewing files that changed from the base of the PR and between aa73a76 and c9db31d.

📒 Files selected for processing (2)

• internal/credstore/credstore.go
• internal/securefile/securefile.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[Vasanthdev2004]

Verified the fsync-before-rename idiom is applied on the right paths and in the right order in both internal/credstore/credstore.go (Store.write) and internal/securefile/securefile.go (writeNewSecretFile): tmp.Sync() is called while the file is still open, then Close, then Rename. Sync errors are checked and the temp file is closed on that path; the existing defer os.Remove(tmpPath) cleans up. os.File.Sync maps to FlushFileBuffers on Windows, so this is correct here too. go build ./..., go vet, and go test on both packages pass, and the PR merges cleanly into current main (its base is a few commits behind but no conflicts). It composes fine with #641, which touches disjoint stores. Approving.