[fix][broker] Support namespace unsubscribe when bundles are unloaded

[zhanghaou]

Motivation

Unsubscribing at namespace/bundle level fails when the target bundle is not loaded by any broker.

This PR makes unsubscribe paths handle unloaded bundles by acquiring ownership when needed and loading topics before unsubscribing.

Modifications

1. Convert namespace/bundle unsubscribe paths to async internal APIs and update v1/v2 admin endpoints to use async response flow.
2. Allow bundle ownership acquisition for unsubscribe operations when bundle is not loaded.
3. Load topics from metadata and ensure they are in memory before removing subscriptions.
4. Add tests to verify unsubscribe works on unloaded bundles (both namespace and bundle scope).

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

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository:

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.65432% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.61%. Comparing base (4cbe124) to head (bec2329).
⚠️ Report is 23 commits behind head on master.

Files with missing lines	Patch %	Lines
...pache/pulsar/broker/admin/impl/NamespacesBase.java	81.81%	6 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #25276      +/-   ##
============================================
- Coverage     72.81%   72.61%   -0.20%     
- Complexity    34134    34555     +421     
============================================
  Files          1959     1968       +9     
  Lines        155543   155954     +411     
  Branches      17741    17766      +25     
============================================
- Hits         113263   113252      -11     
- Misses        33322    33660     +338     
- Partials       8958     9042      +84     

Flag	Coverage Δ
inttests	25.77% <0.00%> (-0.22%)	⬇️
systests	22.35% <60.49%> (-0.19%)	⬇️
unittests	73.57% <87.65%> (-0.21%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
.../org/apache/pulsar/broker/admin/v1/Namespaces.java	53.39% <100.00%> (+0.94%)	⬆️
.../org/apache/pulsar/broker/admin/v2/Namespaces.java	90.81% <100.00%> (+0.22%)	⬆️
...pache/pulsar/broker/admin/impl/NamespacesBase.java	77.81% <81.81%> (+0.21%)	⬆️

... and 112 files with indirect coverage changes

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