test(admin): cover decommission CLI dispatch

[overtrue]

Summary

This PR adds focused CLI integration coverage for the admin decommission workflow introduced with pool decommissioning support.

Background

The lower-level admin client already had route tests for decommission start and cancel, and the CLI parser covered the command shapes. The missing path was the real rc admin decommission binary dispatch through alias resolution, JSON output formatting, and signed admin route selection.

Changes

• Added non-Windows integration tests for admin decommission start, admin decommission status, and the admin decom cancel alias.
• Asserted the generated JSON payloads and captured admin API method, route, and query strings.
• Fixed the recently added rebalance integration test server helper to switch accepted sockets back to blocking mode before reading, preventing a race seen during the full pre-commit run.

Validation

• cargo fmt --all --check
• cargo test -p rustfs-cli --test admin_decommission
• cargo test -p rustfs-cli --test admin_rebalance --test admin_decommission
• make pre-commit

[Copilot]

Pull request overview

Adds CLI integration coverage for the rc admin decommission workflow (including the decom alias) and hardens the admin test TCP server helper to avoid a nonblocking-read race during full test runs.

Changes:

• Added new non-Windows integration tests covering admin decommission start, admin decommission status, and admin decom cancel, asserting JSON output and the dispatched admin route/method/query.
• Fixed the admin_rebalance test server helper by switching the accepted socket back to blocking mode before reading the request.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
crates/cli/tests/admin_rebalance.rs	Ensures the accepted TCP stream is blocking before request parsing to prevent flaky nonblocking reads.
crates/cli/tests/admin_decommission.rs	New integration tests verifying CLI dispatch/aliasing and JSON output for the decommission admin workflow.

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