Improve CLI error message for unknown options to dask worker (#9094)

[ernestprovo23]

Summary

Improves the error message users see when they pass an unknown option or stray positional argument to dask worker (and dask scheduler). Fixes #9094.

Motivation

dask_worker's click group is configured with ignore_unknown_options=True, so any unrecognized token gets passed through into preload_argv and is then caught in validate_preload_argv (distributed/preloading.py).

For dash-prefixed tokens the existing code already raises click.NoSuchOption, which produces a helpful "No such option: --foo" message. But for any non-dash stray value the user just got:

Error: Got unexpected extra arguments: (foo bar)

with no hint that --help lists the valid options or that --preload consumes everything to its right. After #9240 removed the deprecated --nprocs, the original MCVE in #9094 now hits the dash-prefixed branch and produces a good message — but the catch-all message remained the same misleading wall.

Changes

• distributed/preloading.py — extended the click.UsageError message in validate_preload_argv to point users at --help and explain the --preload ordering rule. No structural changes; the NoSuchOption branch is untouched.
• distributed/cli/tests/test_dask_worker.py — two new CliRunner tests:
  • test_unknown_option_reports_no_such_option — regression guard for the dash-prefixed branch.
  • test_stray_positional_hints_at_help_and_preload — asserts the improved message mentions --help and --preload.

Net diff: +26 / -1 across two files.

Testing

Repro on main after #9240:

$ dask worker tcp://127.0.0.1:8786 foo bar
Error: Got unexpected extra arguments: (foo bar)

After this PR:

$ dask worker tcp://127.0.0.1:8786 foo bar
Error: Got unexpected extra arguments: (foo bar). If you intended these as
values for a --preload module, list --preload before them. Otherwise check
spelling -- see '--help' for valid options.

Dash-prefixed unknowns still produce the click-native message:

$ dask worker tcp://127.0.0.1:8786 --nprocs 1
Error: No such option: --nprocs

Local runs:

• pytest distributed/cli/tests/test_dask_worker.py -m "not slow" — 31 passed, 2 skipped (uvloop missing), 0 failed.
• New tests pass.
• pre-commit run --files distributed/preloading.py distributed/cli/tests/test_dask_worker.py — ruff, black, mypy all clean.

Notes:

• validate_preload_argv is also imported by distributed/cli/dask_scheduler.py, so the improved message benefits both entry points. Behavior is unchanged.
• No touching of ignore_unknown_options=True in cli/dask_worker.py — that's a deeper refactor and out of scope for the message fix requested in Dask worker CLI error message might be misleading #9094.

Fixes #9094.

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    31 files  +    1      31 suites  +1   10h 45m 18s ⏱️ + 43m 53s
 4 082 tests +    2   3 969 ✅ +    6    113 💤 +  1  0 ❌  - 3 
59 249 runs  +2 669  56 645 ✅ +2 553  2 604 💤 +122  0 ❌  - 4 

Results for commit a8b8572. ± Comparison against base commit bcad953.

This pull request skips 1 test.

distributed.deploy.tests.test_ssh ‑ test_defer_to_old