chore: propagate RuntimeException from DelayedReadContext (#3150)

Propagate the underlying cause of the ExecutionException as
a RuntimeException directly, if this is already a RuntimeException.
This keeps the behavior of multiplexed sessions and regular
sessions equal, regardless whether the Delayed... route is
taken for multiplexed sessions, or the direct route is used.

Fixes #3132

Author: olavloite

google-cloud-spanner/src/main/java/com/google/cloud/spanner/DelayedReadContext.java

@@ -44,6 +44,11 @@ T getReadContext() {
     try {
       return this.readContextFuture.get();
     } catch (ExecutionException executionException) {
+      // Propagate the underlying exception as a RuntimeException (SpannerException is also a
+      // RuntimeException).
+      if (executionException.getCause() instanceof RuntimeException) {
+        throw (RuntimeException) executionException.getCause();
+      }
       throw SpannerExceptionFactory.asSpannerException(executionException.getCause());
     } catch (InterruptedException interruptedException) {
       throw SpannerExceptionFactory.propagateInterrupt(interruptedException);