test: Fix flaky test TestServerStreaming_ClientCallSendMsgTwice in end2end_test.go.

[Pranjali-2501]

Fixes: #8993

This PR will fix the flaky test TestServerStreaming_ClientCallSendMsgTwice introduced in PR #8385 .

The test expected a single call to stream.SendMsg() to return a codes.Canceled error immediately after the stream's context was marked as done.

Solution: Introduced a retry loop with a timeout for the stream.SendMsg() call. Instead of testing a single invocation, the server handler now continuously attempts to send a message until it either receives an error or timeout expired.

Successfully run the test on forge for 1 million times without any flake.

RELEASE NOTES: N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.99%. Comparing base (ccc8d7b) to head (af59c6d).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9004   +/-   ##
=======================================
  Coverage   82.99%   82.99%           
=======================================
  Files         411      411           
  Lines       32917    32929   +12     
=======================================
+ Hits        27319    27330   +11     
- Misses       4196     4197    +1     
  Partials     1402     1402           

see 28 files with indirect coverage changes

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[Pranjali-2501]

/gemini review

[gemini-code-assist]

Code Review

This pull request modifies the TestServerStreaming_ClientCallSendMsgTwice end-to-end test. The change introduces a polling mechanism to repeatedly call stream.SendMsg until an error is returned or a timeout occurs, ensuring the test correctly waits for the stream's context to be canceled and a codes.Canceled error. The review suggests simplifying the polling logic for better readability by checking context cancellation directly in the loop condition and defining the 10 * time.Millisecond sleep duration as a named constant to improve clarity and maintainability.

[arjan-bal]

LGTM