Use writeFileAtomic in Distribution.Client.Config and remove .tmp workaround

[Copilot]

This updates config-file persistence in Distribution.Client.Config to use atomic writes directly to the target path, instead of a fixed sibling *.tmp file and rename sequence. It also aligns unit tests with the new behavior and adds a focused concurrency regression test for config updates.

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Template Α: This PR modifies behaviour or interface

• Config write path hardening

  • writeConfigFile now:
    • ensures parent directory exists
    • writes directly via writeFileAtomic file ...
  • removed ad-hoc file <.> "tmp" + renameFile flow

• UserConfig unit tests

  • added concurrentUpdateConfig to exercise concurrent userConfigUpdate calls and assert no exceptions
  • removed teardown cleanup for configFile ++ ".tmp" (no longer applicable)

writeConfigFile file comments vals = do
  createDirectoryIfMissing True (takeDirectory file)
  writeFileAtomic file $
    LBS.fromStrict . toUTF8BS $
      explanation ++ showConfigWithComments comments vals ++ "\n"

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