* refactor(e2e/kms): replace fixed startup sleeps with KMS readiness probe
Replace 33 hard-coded sleep(3s) / sleep(2s) startup waits in KMS e2e tests
with an active readiness probe (wait_for_kms_ready) that polls the KMS
status endpoint with exponential backoff (200ms→1s, 5s budget).
This cuts per-test startup latency from a fixed 3s to ~200-500ms while
remaining robust against slow CI machines.
Non-startup sleeps (ILM polling loops, fault-recovery detection delays,
test-runner inter-test pauses) are left untouched.
* style: cargo fmt
* fix(kms): use .expect() instead of ? in test functions that return ()
7 call sites of wait_for_kms_ready() used ? in async test functions
that return () instead of Result. Changed to .expect("KMS ready").
* fix(kms): enforce readiness probe deadline
* fix(kms): validate readiness backend status
Zhengchao An