[EXPORTER] Fix Elasticsearch log exporter Export blocking forever on a non-responding client

[om7057]

Fixes #4362.

The synchronous export path waited on its response condition variable with no deadline of its own, trusting the injected HttpClient to always eventually deliver a terminal event via OnResponse or OnEvent. Nothing in the HttpClient interface actually guarantees that: a client is a supported public surface, not a test seam, and one that accepts a request and never calls back (a dead thread, a reused socket, a swallowed error) left Export() blocked for the life of the process, with no way for a caller's Shutdown() to release it either.

Changes

• waitForResponse() now takes an absolute std::chrono::steady_clock::time_point deadline instead of waiting unconditionally, via cv_.wait_until() in place of cv_.wait().
• The deadline is derived from the exporter's own configured response_timeout_ and captured before SendRequest() is called, so it reflects this exporter's own timeout budget rather than whatever the client does with it.
• A deadline that passes without a terminal event leaves the completion state at Pending, which reads as failure, the same outcome a terminal error event already produces today. No successful path changes.

Testing

Added SilentHttpClient/SilentSession test doubles whose SendRequest() never calls back into the handler at all (no OnResponse, no OnEvent), the exact scenario the issue describes. ExportReturnsOnTimeoutWhenClientNeverResponds constructs the exporter with a 1 second response_timeout_ and this client, and asserts Export() returns kFailure rather than hanging.

Verified the test actually catches the regression: reverted the fix locally (kept the test) and reran under a timeout wrapper, the test hung and was killed at exit code 124 instead of passing vacuously. Restored the fix and confirmed all four tests in the file pass, total runtime 1 second (the deadline in the new test), not 30 (the default response_timeout_).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.51%. Comparing base (0d69d3e) to head (385ae3c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4530   +/-   ##
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  Coverage   86.51%   86.51%           
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  Files         525      525           
  Lines       20469    20469           
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  Hits        17707    17707           
  Misses       2762     2762           

Files with missing lines	Coverage Δ
...orters/elasticsearch/src/es_log_record_exporter.cc	47.73% <ø> (ø)

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[mateenali66]

Ran this against the curl client with response_timeout = 1. A server that never replies fails the export at about 1s on both main and this PR, and replies at 990 to 999ms split the same way. So the change only matters for a custom client.

For that case, Export() still calls session->FinishSession() after the deadline (:495). SilentSession::FinishSession returns at once, so the test can't see it. A client built like curl waits there on the transfer (http_operation_curl.cc:523-538), so a dead worker thread still hangs Export(). curl's CancelSession() does not block (http_client_curl.cc:255-262). Should the expired path cancel instead?

The conflict is only CHANGELOG.md.