[vincent-docto]

Fixes #639

Problem

Every endpoint of a client (e.g. MessagesClient.sendMessage, VoiceClient.createCall) is a single
DynamicEndpoint instance shared by all callers. It kept per-request state in two instance fields:

• cachedRequestBodyString: written then read back inside makeRequest. Under concurrent calls,
  one thread can send the body another thread just wrote. The API still answers 2xx, so the failure is silent.
• cachedRequestBody: used by parseResponse to update the request object from the response. Under
  concurrency, a response can be written into another caller's request object.

parseResponse also reset both fields in a finally, so one thread could wipe another's state, and
a failed HTTP call never reset them at all.

Fix

Remove both fields. The request is now passed to response parsing instead of being stored:

• AbstractMethod: new protected parseResponse(HttpResponse, REQ), called by execute. It delegates
  to the existing parseResponse(HttpResponse) by default, so existing subclasses and the public API
  are unchanged.
• DynamicEndpoint: overrides the new method. The single-argument parseResponse(HttpResponse) is kept
  and delegates with a null request. rawRequest on VonageApiResponseException is now rebuilt from
  the request on failure, so it no longer needs the cached string.

No behaviour change for single-threaded callers.

Tests

• testConcurrentRequestsKeepTheirOwnBody: 8 threads x 500 iterations through one shared endpoint,
  checking each HTTP entity matches its own request. Fails on main (23 mixed bodies in my run), passes with this change.
• testResponseIsParsedIntoItsOwnRequest: a response is parsed into the request it belongs to.
• Full suite passes (1468 tests).

Notes

This does not change the README FAQ guidance on thread safety. Other state in the SDK may still not be
thread-safe. I only audited DynamicEndpoint.

Contribution Checklist

• Unit tests!
• Updated CHANGELOG.md
• My name is in CONTRIBUTORS.md

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.07%. Comparing base (6564a9e) to head (0d5b561).

Files with missing lines	Patch %	Lines
...c/main/java/com/vonage/client/DynamicEndpoint.java	93.75%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #640      +/-   ##
============================================
- Coverage     99.07%   99.07%   -0.01%     
- Complexity     3660     3661       +1     
============================================
  Files           549      549              
  Lines          8076     8073       -3     
  Branches        446      446              
============================================
- Hits           8001     7998       -3     
  Misses           55       55              
  Partials         20       20              

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