Standardize OkHttpHttpSender shutdown to await executor termination

[thswlsqls]

Fixes #8280

Description

• Align OkHttpHttpSender.shutdown() with the sibling OkHttpGrpcSender.shutdown() in the same package, which Standardize sender shutdown implementations #8280 designates as the reference implementation.
• Order the shutdown steps consistently: dispatcher().cancelAll(), then connectionPool().evictAll(), then (for a managed executor) executorService.shutdownNow().
• For a managed executor, await termination instead of returning immediately: a daemon okhttp-shutdown thread calls awaitTermination(5, SECONDS), logs a WARNING on timeout, and completes the returned CompletableResultCode. A non-managed executor still returns CompletableResultCode.ofSuccess().
• Internal transport package (io.opentelemetry.exporter.sender.okhttp.internal); no public API change, no apidiff, no CHANGELOG entry.

Testing done

• Added OkHttpHttpSenderTest shutdown tests mirroring OkHttpGrpcSenderTest: shutdown_CompletableResultCodeShouldWaitForThreads, shutdown_NonManagedExecutor_ReturnsImmediately, shutdown_ExecutorDoesNotTerminateInTime_LogsWarningButSucceeds, shutdown_InterruptedWhileWaiting_StillSucceeds.
• ./gradlew :exporters:sender:okhttp:check passed (5 tests, 0 failures).

[codecov]

Codecov Report

❌ Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.76%. Comparing base (824334c) to head (23ac0b3).

Files with missing lines	Patch %	Lines
...orter/sender/okhttp/internal/OkHttpHttpSender.java	73.33%	3 Missing and 1 partial ⚠️

❌ Your patch check has failed because the patch coverage (73.33%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8495      +/-   ##
============================================
- Coverage     78.77%   78.76%   -0.01%     
- Complexity     8579     8580       +1     
============================================
  Files          1009     1009              
  Lines         28993    29006      +13     
  Branches       3599     3600       +1     
============================================
+ Hits          22839    22848       +9     
- Misses         5311     5314       +3     
- Partials        843      844       +1     

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