test(admin): cover expand command dispatch

[overtrue]

Related Issue(s)

None.

Problem Background and User Impact

PR #180 added rc admin expand and the scale alias as an expansion-oriented wrapper around the existing rebalance workflow. The help and parser coverage prove the commands are discoverable, but they did not exercise the binary command path against an admin endpoint.

Root Cause Summary

The new command reuses the rebalance implementation with expansion-specific start and stop messages. Without an execution-level test, a dispatch regression could still parse successfully while failing to call the expected /rebalance/* admin routes or returning the wrong JSON operation message.

Solution Overview

This PR adds focused integration coverage for the expansion command path. A local one-shot HTTP server captures the signed admin request while the CLI uses an RC_HOST_* environment alias. The tests verify that admin scale start reaches /rustfs/admin/v3/rebalance/start and that admin expand stop reaches /rustfs/admin/v3/rebalance/stop, while also asserting the expansion-specific JSON output.

Test Status

• cargo test -p rustfs-cli --test admin_expand
• cargo fmt --all --check
• git diff --check
• make pre-commit

[Copilot]

Pull request overview

Adds an integration test to exercise the rc admin expand/rc admin scale execution path against an admin HTTP endpoint, ensuring the command dispatch reaches the expected rebalance routes and returns the expansion-specific JSON messages.

Changes:

• Introduces a one-shot local HTTP server that captures the admin request method + target path.
• Adds coverage verifying admin scale start hits /rustfs/admin/v3/rebalance/start and returns the “Expansion rebalance started successfully” JSON payload (including operation id).
• Adds coverage verifying admin expand stop hits /rustfs/admin/v3/rebalance/stop and returns the “Expansion rebalance stopped successfully” JSON payload (without an id).

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