[fix][broker] Fix stuck chunks in SharedConsumerAssignor permit tracking

[merlimat]

Motivation

MessageChunkingSharedTest.testMultiConsumers is flaky — it hangs intermittently and was bumped from 3s to 15s in #25475 to mask the symptom. The root cause is a real correctness bug in SharedConsumerAssignor that can cause chunked messages on Shared subscriptions to get permanently stuck in the redelivery queue, producing partial reassemblies on the consumer that never complete.

Bug

In SharedConsumerAssignor.getConsumerForUuid:

consumerToPermits.put(consumer, currentAvailablePermits - 1);

currentAvailablePermits is the assignor loop's local tracker for the loop's defaultConsumer. But when a cache hit on uuidToConsumer redirects a chunk to a different consumer, consumer and defaultConsumer are not the same. The line above then writes the loop consumer's tracker into the target consumer's slot, corrupting consumerToPermits and making the assignor's per-consumer accounting drift over-optimistic.

The chain that produces a stuck chunk:

1. Over-allocation makes consumerToPermits[X] claim more permits than X actually has.
2. The assignor allocates more entries to X than X can take.
3. sendChunkedMessagesToConsumers trims via Math.min(consumer.getAvailablePermits(), entryList.size()) and pushes the excess to the redelivery queue.
4. For a last-chunk entry that gets trimmed, the assignor already removed uuidToConsumer[uuid] on line 145.
5. When that last-chunk entry is re-read from redelivery, the assignor sees no cache entry and chunkId != 0, so it returns null — and the entry goes right back to redelivery. Forever.
6. The consumer holds chunks 0..N-2 in chunkedMessagesMap waiting for chunk N-1 that will never arrive; the partial message never reassembles, never acks.

When this happens the test hangs until TestNG's retry kicks in 30s later, masking the failure as a "slow run."

Modifications

getConsumerForUuid now decrements the target consumer's permits using the value it just read from consumerToPermits, not the caller's local tracker. The currentAvailablePermits parameter is no longer needed and was removed.

Verification

Locally, before the fix the test was bimodal: either ~0.5s or ~30s (TestNG retry threshold). After the fix, 0/30 runs were slow:

	Before fix	After fix
Slow runs (>5s)	2/10	0/30

Other chunking suites (MessageChunkingTest, MessageChunkingSharedTest, MessageChunkingDeduplicationTest, SharedConsumerAssignorTest) still pass.

Test plan

• mvn test -pl pulsar-broker -Dtest='MessageChunkingSharedTest' — passes
• mvn test -pl pulsar-broker -Dtest='SharedConsumerAssignorTest' — passes
• Stress run: 30 iterations of MessageChunkingSharedTest.testMultiConsumers — 0 slow / 0 failed

Matching PR types in the table

• area/broker