fix: return error on non-ErrNotExist stat failures in Tar.Sync()

[Lidang-Jiang]

Summary

• Fix silent misclassification of non-ErrNotExist stat errors in Tar.Sync() (internal/sync/tar.go)
• Add unit tests for the three stat outcomes: exists, not-exists, and permission error

The previous two-way branch only checked for fs.ErrNotExist. Any other os.Stat error (e.g. EACCES, EIO) fell through to the else clause, silently treating the path as copyable. This masked real errors and caused failures downstream with the original cause lost.

The fix restructures the condition into a three-way branch, following the pattern already used by entriesForPath() in the same file.

Fixes #13654

Before / After

Before (buggy behavior)

Non-ErrNotExist stat errors are silently treated as "copy":

=== Old (buggy) logic ===
  /tmp/before_test_existing.txt → COPY (err=<nil>)
  /no/such/path → DELETE (not exist)
  /tmp/before_test_noaccess/secret.txt → COPY (err=stat /tmp/before_test_noaccess/secret.txt: permission denied)

The path with a permission error is incorrectly added to the copy list instead of surfacing the error.

After (fixed behavior)

Permission errors are now properly returned:

=== New (fixed) logic ===
  /tmp/before_test_existing.txt → COPY
  /no/such/path → DELETE
  /tmp/before_test_noaccess/secret.txt → ERROR: stat /tmp/before_test_noaccess/secret.txt: permission denied

Unit test output:

=== RUN   TestSync_ExistingPath
--- PASS: TestSync_ExistingPath (0.00s)
=== RUN   TestSync_NonExistentPath
--- PASS: TestSync_NonExistentPath (0.00s)
=== RUN   TestSync_StatPermissionError
--- PASS: TestSync_StatPermissionError (0.00s)
=== RUN   TestSync_MixedPaths
--- PASS: TestSync_MixedPaths (0.00s)
PASS
ok  	github.com/docker/compose/v5/internal/sync	0.005s

Lint:

$ golangci-lint run ./internal/sync/...
0 issues.

Test plan

• TestSync_ExistingPath — existing host path is correctly added to copy list
• TestSync_NonExistentPath — missing host path triggers rm -rf delete command
• TestSync_StatPermissionError — EACCES error is returned immediately (skipped on root/Windows)
• TestSync_MixedPaths — mix of existing and missing paths handled correctly in one call
• golangci-lint run ./internal/sync/... — 0 issues