Reset subscriberRef in prepare

[dagnir]

Description

Fix a bug in EventStreamAsyncResponseTransformer where the subscriber ref is not
reset between calls to prepare(), causing retries to fail when the
EventStreamResponseHandler attempts to subscribe to a new stream on a request
retry.

Motivation and Context

Testing

New unit test.

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed
• A short description of the change has been added to the CHANGELOG
• My change is to implement 1.11 parity feature and I have updated LaunchChangelog

License

• I confirm that this pull request can be released under the Apache 2 license

[codecov-io]

Codecov Report

Merging #1255 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #1255      +/-   ##
============================================
- Coverage     70.14%   70.13%   -0.01%     
  Complexity      188      188              
============================================
  Files           776      776              
  Lines         24121    24122       +1     
  Branches       1799     1799              
============================================
  Hits          16919    16919              
- Misses         6419     6420       +1     
  Partials        783      783

Flag	Coverage Δ	Complexity Δ
#unittests	70.13% <100%> (-0.01%)	188 <0> (ø)

Impacted Files	Coverage Δ	Complexity Δ
...entstream/EventStreamAsyncResponseTransformer.java	86.55% <100%> (+0.07%)	0 <0> (ø)	⬇️
...o/netty/internal/http2/HttpOrHttp2ChannelPool.java	78.49% <0%> (-1.08%)	0% <0%> (ø)
...on/awssdk/services/kinesis/KinesisRetryPolicy.java	85.71% <0%> (ø)	0% <0%> (ø)	⬇️

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