chore(cli): point error messages at the positional 'file'/'object', not --file/--object

[FileSystemGuy]

Summary

The data-access protocol is a positional (mlpstorage_py/cli/common_args.py:329 — add_storage_type_arguments registers it with choices=['file', 'object']). The old --file / --object flag form was removed when #376 was fixed.

Several user-facing error messages, help strings, and comments still tell users to type --file --o-direct or similar. A user who follows those hints types the suggested string and hits:

```
argparse: error: unrecognized arguments: --file
```

This PR fixes the user-visible wording in three CLI files and points operators at the actual positional with a copy-pasteable example command form.

Changes

• mlpstorage_py/cli/training_args.py — --o-direct rejection messages (both branches) and the --o-direct argparse help text.
• mlpstorage_py/cli/checkpointing_args.py — same pair.
• mlpstorage_py/cli/kvcache_args.py — the "only POSIX file storage" rejection message.

Before / after examples:

```

• ERROR: --o-direct is incompatible with --object.
• ...
• Use --file --o-direct for O_DIRECT local I/O.

• ERROR: --o-direct is incompatible with the 'object' data-access protocol.
• ...
• Use the 'file' positional with --o-direct for O_DIRECT local I/O,
• e.g. `mlpstorage training run file --o-direct`.
  ```

PR #544 introduced two of the affected --o-direct messages most recently; the rest pre-date #376. The flag form remains documented as historical context in cli_parser.py and the test_issue_376_file_arg_conflict.py test — those are intentional records of the removal and not changed here.

Test updates

Test assertions in tests/unit/test_dlio_odirect.py that previously matched on the literal --object / --file substrings are updated to match the new quoted-positional form ('object' / 'file') so they verify the corrected wording.

Test plan

• `pytest tests/unit/test_dlio_odirect.py` — passes (3 assertion updates)
• `pytest tests/unit/test_dlio_odirect.py tests/unit/test_dlio_object_storage.py tests/unit/test_cli.py tests/unit/test_cli_kvcache.py tests/unit/test_benchmarks_kvcache.py tests/unit/test_help_behavior.py` — 369/369 pass

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