feat(admin): add decommission clear command

[cxymds]

Related Issue

None.

Background

rc admin decommission supported start, status, and cancel, but it did not expose RustFS' pool decommission clear operation. Operators need this command to clear failed or canceled decommission metadata by pool, including by pool ID.

Root Cause

The admin API trait and S3 admin client did not include a decommission_clear operation, and the CLI decommission subcommands did not include clear.

Solution

• Add rc admin decommission clear <ALIAS> <POOL> [--by-id].
• Dispatch clear requests to POST /rustfs/admin/v3/pools/clear with the existing pool query shape.
• Add parser, help contract, CLI dispatch, and S3 route coverage.
• Extend the shared admin test server accept deadline to avoid false failures when spawned CLI tests start slowly under local load.

Test Status

• cargo fmt --all --check
• cargo clippy --workspace -- -D warnings
• cargo test --workspace