[forge] reduce encrypted max-load backlog to 4000

Backlog 8000 overran the chain's saturation point (~960 TPS at 100
validators) and triggered an unrecoverable expiration cascade. 4000
sits right at the saturation knee with stable commit throughput.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ibalajiarun

testsuite/forge-cli/src/suites/realistic_environment.rs

@@ -518,7 +518,7 @@ pub(crate) fn realistic_env_max_load_encrypted_test(duration: Duration) -> Forge
     let num_validators = 5;
     let num_fullnodes = 1;
     let num_pfns = 3;
-    let mempool_backlog = 8000;
+    let mempool_backlog = 4000;
 
     let success_criteria = SuccessCriteria::new(15)
         .add_no_restarts()