[kubectl-plugin] Skip out-of-range file modes and close files during log download

[SrikarChittemsetty]

Why are these changes needed?

Two bugs in downloadRayLogFiles (kubectl-plugin/pkg/cmd/log/log.go), reported in #4779.

1. The mode bounds check does nothing. The code checks whether header.Mode fits in an os.FileMode and prints a warning, but has no continue/guard — it falls straight through to os.OpenFile(..., os.FileMode(header.Mode)) and creates the file with a truncated mode. The //nolint:gosec // overflow is guarded by bounds check above comment asserted a guard that was not there, so the linter stayed silent.

Worth noting for reviewers: the fix suggested in the issue (adding a bare continue) would hang. The extraction loop advances the tar reader after the switch:

for !errors.Is(err, io.EOF) {
    switch header.Typeflag { ... }
    header, err = tarReader.Next()   // <- skipped by `continue`
}

so continue would re-read the same header forever. This PR guards the write with an else branch instead, which skips the file without skipping the advance. A regression test asserts the call returns rather than stalling.

2. File descriptors accumulate for the whole download. defer outFile.Close() sat inside the extraction loop, so defer only ran when downloadRayLogFiles returned — every file in the archive stayed open until the entire download finished. Extracting a log archive with many files holds that many descriptors at once.

The per-entry write moves into a writeTarFile helper so the file closes at the end of each entry, and close errors are surfaced rather than discarded.

Related issue number

Closes #4779

Checks

• go build ./..., go vet, and gofmt are clean.
• Extended TestDownloadRayLogFiles coverage with TestDownloadRayLogFilesSkipsOutOfRangeMode, which asserts the out-of-range entry is skipped, that the following valid entry still extracts, and that extraction does not stall. Against the previous behaviour it fails with:

Error: "[- file0.txt - file1.txt]" should have 1 item(s), but has 2

Disclosure: this change was prepared with the assistance of an AI coding agent. The analysis, fix, and tests were verified against the source and the test suite before submitting.