fix: improve driver-side timeout logging with timeout value and pool state

When OperationTimedOutException fires in SpeculativeExecution.onTimeout(),
log the configured driver-side timeout, per-connection in-flight count,
pool pending borrow queue length, and total pool in-flight count.

This helps distinguish between two failure modes:
- Pool contention (requests queued inside the driver before reaching server):
  visible as pool pending borrows > 0
- Server-side slowness (requests reached server but driver gave up first):
  visible as pool pending borrows = 0 with high connection in-flight

To expose the timeout value, ResponseHandler.readTimeoutMillis is widened
from private to package-private. All access remains within
com.datastax.driver.core.

Motivated by CUSTOMER-301 (Whoop): client-side OperationTimedOutException
bursts with no corresponding server-side timeouts, requiring diagnostics
to determine whether the bottleneck is in the driver pool or on the server.

Author: nikagra

driver-core/src/main/java/com/datastax/driver/core/Connection.java

@@ -1887,7 +1887,7 @@ static class ResponseHandler {
     final int streamId;
     final ResponseCallback callback;
     final int retryCount;
-    private final long readTimeoutMillis;
+    final long readTimeoutMillis;
 
     private final long startTime;
     private volatile Timeout timeout;

driver-core/src/main/java/com/datastax/driver/core/RequestHandler.java

@@ -1007,6 +1007,20 @@ public boolean onTimeout(Connection connection, long latency, int retryCount) {
 
       Host queriedHost = current;
 
+      HostConnectionPool pool = manager.pools.get(queriedHost);
+      logger.warn(
+          "[{}] Driver-side timeout waiting for response from {} — "
+              + "configured timeout: {}ms, "
+              + "connection in-flight: {}, "
+              + "pool pending borrows: {}, "
+              + "pool total in-flight: {}",
+          id,
+          connection.endPoint,
+          connectionHandler != null ? connectionHandler.readTimeoutMillis : "unknown",
+          connection.inFlight.get(),
+          pool != null ? pool.pendingBorrowCount.get() : "unknown",
+          pool != null ? pool.totalInFlight.get() : "unknown");
+
       OperationTimedOutException timeoutException =
           new OperationTimedOutException(
               connection.endPoint, "Timed out waiting for server response");