[fix][client] Fail messages immediately in ProducerImpl when in terminal state

[merlimat]

Motivation

When a producer reaches a terminal state (Terminated, Closed, or ProducerFenced), messages entering processOpSendMsg() would be added to pendingMessages with no connection to write them, leaving them stuck until sendTimeout (default 30s).

Add a terminal state check in processOpSendMsg() before adding to the pending queue, failing the message immediately with the appropriate exception. Also fix testTerminateWhilePublishing to close the producer after termination, since the broker's CommandCloseProducer races with CommandSendError responses, preventing the client from learning about topic termination through the normal error path.

Modifications

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 11.76471% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.66%. Comparing base (b171b05) to head (146ef82).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...va/org/apache/pulsar/client/impl/ProducerImpl.java	11.76%	14 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             master   #25317    +/-   ##
==========================================
  Coverage     72.66%   72.66%            
+ Complexity    34651    34244   -407     
==========================================
  Files          1967     1967            
  Lines        156326   156342    +16     
  Branches      17812    17813     +1     
==========================================
+ Hits         113591   113612    +21     
+ Misses        33671    33638    -33     
- Partials       9064     9092    +28     

Flag	Coverage Δ
inttests	25.76% <5.88%> (+0.05%)	⬆️
systests	22.29% <11.76%> (-0.19%)	⬇️
unittests	73.63% <11.76%> (+0.01%)	⬆️

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

Files with missing lines	Coverage Δ
...va/org/apache/pulsar/client/impl/ProducerImpl.java	83.30% <11.76%> (-0.90%)	⬇️

... and 97 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.