[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 83.46%. Comparing base (eec1b36) to head (dabaa7b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4530   +/-   ##
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  Coverage   83.46%   83.46%           
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  Files         521      521           
  Lines       20412    20412           
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  Hits        17034    17034           
  Misses       3378     3378           

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

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