Add known-key completion for neofs-adm fschain set-config

[Copilot]

neofs-adm fschain set-config already accepts a fixed set of well-known keys without --force, but it did not expose them via shell completion. This aligns its autocomplete behavior with set-policy and makes the supported key set discoverable at the CLI.

• Completion support

  • Add ValidArgsFunction for fschain set-config
  • Return the well-known config keys as key= suggestions
  • Preserve ShellCompDirectiveNoSpace so values can be typed immediately after completion

• Shared well-known key list

  • Centralize the completion candidates in a single list derived from the existing supported config keys
  • Keep autocomplete aligned with the keys already handled by parseConfigPair

• Focused coverage

  • Add a unit test for the set-config completion output and directive

ValidArgsFunction: func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
	return setConfigKnownArgs, cobra.ShellCompDirectiveNoSpace
}

• Fixes Improve autocomplete for neofs-adm morph set-config #1901

[roman-khimov]

@copilot, add changelog entry

[Copilot]

@copilot, add changelog entry

Addressed in 6e5d6f8.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 31.16%. Comparing base (3a3655b) to head (3751df7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4177      +/-   ##
==========================================
- Coverage   31.17%   31.16%   -0.01%     
==========================================
  Files         673      673              
  Lines       40778    40779       +1     
==========================================
- Hits        12712    12710       -2     
- Misses      28066    28069       +3     

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