fix: ensure subsequent slots execute after PriorityWaitException in FlowSlot

[daguimu]

Problem

When FlowSlot.checkFlow() throws PriorityWaitException (for prioritized requests that pass flow control by waiting for a future token), fireEntry() is never called. This causes all subsequent slots in the chain to be skipped.

The default slot chain order is:

StatisticSlot(-7000) → ... → FlowSlot(-2000) → DefaultCircuitBreakerSlot(-1500) → DegradeSlot(-1000)

When PriorityWaitException is thrown, DefaultCircuitBreakerSlot and DegradeSlot are never executed. This means circuit breaker checks are bypassed for priority-waited requests — even if a circuit breaker is in the OPEN state, the request passes through unchecked.

Root Cause

In FlowSlot.entry(), checkFlow() is called before fireEntry(). When checkFlow() throws PriorityWaitException, the exception propagates immediately and fireEntry() (which chains to downstream slots) is never reached:

// Before fix
public void entry(...) throws Throwable {
    checkFlow(...);         // throws PriorityWaitException
    fireEntry(...);         // NEVER reached
}

Fix

Catch PriorityWaitException in FlowSlot.entry(), execute downstream slots via fireEntry(), then re-throw the exception for StatisticSlot to handle:

// After fix
public void entry(...) throws Throwable {
    try {
        checkFlow(...);
    } catch (PriorityWaitException ex) {
        fireEntry(...);     // Execute downstream slots (circuit breaker, degrade)
        throw ex;           // Re-throw for StatisticSlot
    }
    fireEntry(...);
}

If a downstream slot throws BlockException (e.g., circuit breaker is OPEN), the BlockException propagates instead of PriorityWaitException, and StatisticSlot handles it as a block.

Tests Added

• testPriorityWaitExceptionStillFiresSubsequentSlots: Verifies that downstream slots are executed when PriorityWaitException is thrown, and the exception is still re-thrown.
• testPriorityWaitWithDownstreamBlockPropagatesBlockException: Verifies that if a downstream slot throws BlockException after a priority wait, the BlockException takes precedence.

All 225 tests in sentinel-core pass.

Impact

Only affects FlowSlot.entry() behavior when PriorityWaitException is thrown (prioritized QPS-based flow control with token waiting). Normal flow control (pass/block) is unchanged.

Fixes #3604