Bound OkHttp sender dispatchers and surface rejections

[ADITYA-CODE-SOURCE]

Fix#8316.

Why

• OkHttp sender dispatchers were backed by an executor with Integer.MAX_VALUE max threads.
• Rejected dispatcher execution should surface as exporter failure instead of escaping the sender.

What

• cap the managed OkHttp dispatcher executor at OkHttp's default maxRequests bound
• apply the same dispatcher request limits when a custom executor is supplied
• catch synchronous RejectedExecutionException from enqueue(...) in both HTTP and gRPC senders
• add focused tests for dispatcher bounds and rejected execution handling

Testing

• ./gradlew --no-daemon --max-workers=1 -Dorg.gradle.jvmargs="-Xmx512m -XX:MaxMetaspaceSize=192m" :exporters:sender:okhttp:spotlessCheck
• ./gradlew --no-daemon --max-workers=1 -Dorg.gradle.jvmargs="-Xmx768m -XX:MaxMetaspaceSize=256m" :exporters:sender:okhttp:test --tests io.opentelemetry.exporter.sender.okhttp.internal.OkHttpUtilTest --tests io.opentelemetry.exporter.sender.okhttp.internal.OkHttpHttpSenderTest --tests io.opentelemetry.exporter.sender.okhttp.internal.OkHttpGrpcSenderTest (fails locally on this Windows machine because Gradle worker JVMs hit paging-file / native memory limits before the module finishes compiling; pushing to CI for full verification)

[codecov]

Codecov Report

❌ Patch coverage is 87.09677% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.13%. Comparing base (b1b563b) to head (ee2e0c1).
⚠️ Report is 46 commits behind head on main.

Files with missing lines	Patch %	Lines
...orter/sender/okhttp/internal/OkHttpGrpcSender.java	84.61%	2 Missing ⚠️
...orter/sender/okhttp/internal/OkHttpHttpSender.java	84.61%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8422      +/-   ##
============================================
+ Coverage     90.83%   91.13%   +0.30%     
+ Complexity     7927     7764     -163     
============================================
  Files           895      881      -14     
  Lines         23872    23419     -453     
  Branches       2378     2331      -47     
============================================
- Hits          21683    21344     -339     
+ Misses         1446     1380      -66     
+ Partials        743      695      -48     

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