PR #21231 made user topic creation rely on system topic `__change_event` if the user is enabling `topicLevelPoliciesEnabled`.

It will introduce a race condition with namespace bundle unloading. All creating topics want to create `__change_event` reader but constantly fail by namespace-bundle inactive and retry mechanism. Unfortunately, the namespace-bundle unloading operation is waiting for all the topics to be completed and then release inactive status. Therefore, they will be stuck in a deadlock until one gets a timeout.

- Get the topic policy before loading.

…21445)

### Motivation

PR apache#21231 made user topic creation rely on system topic `__change_event` if the user is enabling `topicLevelPoliciesEnabled`.

It will introduce a race condition with namespace bundle unloading. All creating topics want to create `__change_event` reader but constantly fail by namespace-bundle inactive and retry mechanism. Unfortunately, the namespace-bundle unloading operation is waiting for all the topics to be completed and then release inactive status. Therefore, they will be stuck in a deadlock until one gets a timeout.


### Modifications

- Get the topic policy before loading.

…#21565)

Cherry-pick #21445 to branch-3.0

…#21567)

Co-authored-by: Jiwe Guo <technoboy@apache.org>

…#21566)

…21445) (apache#21565)

Cherry-pick apache#21445 to branch-3.0

…21445) (apache#21567)

Co-authored-by: Jiwe Guo <technoboy@apache.org>

…21445)

PR apache#21231 made user topic creation rely on system topic `__change_event` if the user is enabling `topicLevelPoliciesEnabled`.

It will introduce a race condition with namespace bundle unloading. All creating topics want to create `__change_event` reader but constantly fail by namespace-bundle inactive and retry mechanism. Unfortunately, the namespace-bundle unloading operation is waiting for all the topics to be completed and then release inactive status. Therefore, they will be stuck in a deadlock until one gets a timeout.

- Get the topic policy before loading.

…21445)

PR apache#21231 made user topic creation rely on system topic `__change_event` if the user is enabling `topicLevelPoliciesEnabled`.

It will introduce a race condition with namespace bundle unloading. All creating topics want to create `__change_event` reader but constantly fail by namespace-bundle inactive and retry mechanism. Unfortunately, the namespace-bundle unloading operation is waiting for all the topics to be completed and then release inactive status. Therefore, they will be stuck in a deadlock until one gets a timeout.

- Get the topic policy before loading.

…licies before loading topics

### Motivation

apache#21445 introduces an implicit
requirement that if the topic policies cannot be loaded,
`BrokerService#getTopic` will fail and then the client will retry
loading the topic.

It could break some existing usages like the tests in the C++
client: apache/pulsar-client-cpp#394

This change is applied only to avoid the race condition when unloading a
namespace bundle. However, the client should not fail if the topic-level
policies are not available.

### Modifications

If the topic policies cannot be loaded due to a non-retryable error, we
should not fail the `getTopic`. For retryable errors, the client will
still retry until the broker gets the topic-level policies successfully
after some attempts.

Modify `TokenAuthenticatedProducerConsumerTest` to protect the change.