[fix][broker] Fix backlog clearing for unloaded namespace bundles

[zhanghaou]

Fixes #20706

Motivation

Clearing backlog for a namespace/bundle should work even when the bundle is not loaded on any broker.

Modifications

1. Convert namespace backlog clear paths to async internal methods and update v1/v2 admin resources to use async-response flow consistent with existing patterns (e.g. getMaxProducersPerTopic).
2. Ensure bundle backlog clearing can operate when bundles are unloaded by enumerating persistent topics from metadata and loading topics as needed.
3. Add tests to verify backlog clearing on unloaded bundles (all subscriptions and per-subscription cases).

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

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• doc-not-needed
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