DefaultTransactionPrehandler busy-waits with no yield while waiting for latest immutable state

**Problem**
In `DefaultTransactionPrehandler.prehandleApplicationTransactions()` (`platform-sdk/consensus-transaction-handling/src/main/java/org/hiero/consensus/transaction/handling/internal/DefaultTransactionPrehandler.java`), the code busy-waits for the latest immutable state with no yield, sleep, backoff, or timeout:

```java
while (latestImmutableState == null) {
latestImmutableState = signedStateNexus.getState(RESERVATION_LABEL);
}
```

`signedStateNexus.getState()` is documented as possibly returning `null` (implementation detail of the locking mechanism within the supplier). If it returns null for an extended period every pre-handle worker thread burns 100% CPU in a tight spin with no log message or metric, stalling the pre-handle pipeline.

**Trigger is internal** (supplier returning null due to locking/timing), not attacker-controlled input

**Proposed fix**
- Add `Thread.yield()` inside the spin loop to release the CPU between polls

**Reference:** originally flagged as VLN-620 (hedera-security-issues#708) — confirmed not a security issue, but a real liveness/CPU-burn bug. Fix scope intentionally kept minimal (yield only) rather than the full bounded-retry/timeout/metric remediation from the original finding.