fix: skip non-writable directory check in TestPreCheckConfig when running as root

[Copilot]

What problem does this PR solve?

Issue Number: close #xxx

TestPreCheckConfig was flaky on CI because the non-writable directory subtest (0o400 perms) expects PreCheckConfig to return an error, but Linux root bypasses DAC permission checks — IsDirReadWritable succeeds unconditionally as root, causing a spurious nil error result.

What is changed and how it works?

• Wraps the non-writable directory subtest in os.Getuid() != 0 guard so it's skipped when the test process runs as root, where the assumption "can't write to a 0o400 dir" simply doesn't hold.

if os.Getuid() != 0 {
    baseDir = filepath.Join(dir, "not-writable")
    require.NoError(t, os.MkdirAll(baseDir, 0o400))
    err = PreCheckConfig(resModel.LocalFileConfig{BaseDir: baseDir})
    require.Error(t, err)
    require.Regexp(t, ".*ErrLocalFileDirNotWritable.*", err)
}

Check List

Tests

• Unit test

Questions

Will it cause performance regression or break compatibility?

No.

Do you need to update user documentation, design documentation or monitoring documentation?

No.

Release note

None

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Flaky unit test: TestPreCheckConfig in engine/pkg/externalresource/internal/local</issue_title>
<issue_description>Summary
TestPreCheckConfig in engine/pkg/externalresource/internal/local is flaky on master. The test intermittently expects an error but receives nil.

Failure Log

=== FAIL: engine/pkg/externalresource/internal/local TestPreCheckConfig (0.00s)
    file_manager_test.go:285:
[2026/03/10 17:43:36.414 +08:00] [INFO] [file_manager.go:70] ["Created directory for local file resource"] [resource-name=resource-1-1] [path=/tmp/TestFileManagerManyWorkers3730084022/001/worker-1/7265736f757263652d312d31]
[2026/03/10 17:43:36.414 +08:00] [INFO] [file_manager.go:221] ["Local resource has been removed"] [resource-id=resource-1]
        Error Trace:    /home/prow/go/src/github.com/pingcap/tiflow/engine/pkg/externalresource/internal/local/file_manager_test.go:285
        Error:          An error is expected but got nil.
        Test:           TestPreCheckConfig
[2026/03/10 17:43:36.414 +08:00] [INFO] [file_manager.go:70] ["Created directory for local file resource"] [resource-name=resource-1-2] [path=/tmp/TestFileManagerManyWorkers3730084022/001/worker-1/7265736f757263652d312d32]
[2026/03/10 17:43:36.415 +08:00] [INFO] [file_manager.go:70] ["Created directory for local file resource"] [resource-name=resource-2-1] [path=/tmp/TestFileManagerManyWorkers3730084022/001/worker-2/7265736f757263652d322d31]

References

• encountered in PR: sync-diff-inspector: improve checksum SQL with configurable algorithm and float handling #12533
• CI build: https://prow.tidb.net/view/gs/prow-tidb-logs/pr-logs/pull/pingcap_tiflow/12533/wip-pull-unit-test-engine/2031303054317326336

Notes

• Observed on 2026-03-10.
  </issue_description>

<agent_instructions>fix the bug and fix it.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Flaky unit test: TestPreCheckConfig in engine/pkg/externalresource/internal/local #12547

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/test wip-pull-unit-test-engine wip-pull-unit-test-dm

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wip-pull-unit-test-engine	0ed5e0d	link	true	/test wip-pull-unit-test-engine
wip-pull-unit-test-dm	0ed5e0d	link	true	/test wip-pull-unit-test-dm

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