test(s3): cover heal request contracts

[overtrue]

Related issue

Automated test-gap detection for recent RustFS CLI changes. No user-facing bug report is linked.

Problem and user impact

The recent admin heal API alignment changed the RustFS admin endpoints used by heal status, start, and stop operations. Existing tests covered helper serialization and mapping, but they did not verify the actual HTTP request method, path, query string, or request body emitted by AdminClient.

Root cause

The request contract lived across small helpers plus AdminClient methods, so a future regression in method selection, encoded target construction, force-stop query parameters, or JSON body emission could pass the existing unit tests.

Solution

Add loopback HTTP tests for heal_status, heal_start, and heal_stop. The tests capture one signed admin request and assert the RustFS admin endpoint, method, query, and body shape.

Validation

• cargo test -p rc-s3 admin::tests::test_heal_ -- --nocapture
• make pre-commit