OkHttp gRPC sender retries are rejected by a bounded executor #8964

Describe the bug
Since #8854, OkHttpGrpcSender starts each retry by calling Call.enqueue() from inside the OkHttp callback of the failed attempt. That dispatcher thread is still busy, so with a bounded executor every retry is rejected and the export fails with InterruptedIOException: executor rejected instead of being retried.

Steps to reproduce

1. Build an OTLP gRPC exporter with retry enabled (the default) and setExecutorService(new ThreadPoolExecutor(1, 1, 0, TimeUnit.SECONDS, new SynchronousQueue<>())).
2. Point it at a closed localhost port.
3. Export once.

What did you expect to see?
A retry after backoff, then the ConnectException. Before #8854, RetryInterceptor retried on the same thread, as OkHttpHttpSender and JdkHttpSender still do.

What did you see instead?
The retry fails with java.io.InterruptedIOException: executor rejected in OkHttpGrpcSender.sendAttempt() → enqueue() and never reaches the network.

What version and what artifacts are you using?
Artifacts: opentelemetry-exporter-otlp, opentelemetry-exporter-sender-okhttp
Version: 1.67.0 / main @ b32298a
How did you reference these artifacts? Gradle

Environment
Compiler: Temurin 21
OS: N/A

Additional context
I have a small fix that runs retries synchronously on the current thread. Happy to adjust if maintainers prefer a different execution model (e.g. a JdkHttpSender-style loop).