Core-LB-G08: Add overload recovery scenario tests

[RicheyWorks]

Overall Classification: WARN
Campaign: Core LoadBalancer Reliability Contract Campaign
Goal: Core-LB-G08
Scope: docs/test-only overload and recovery scenario coverage

Summary:

• Adds local deterministic overload and recovery scenario tests for the core LoadBalancer facade.
• Covers capacity exhaustion, all-unhealthy degradation, recovered healthy candidate re-entry, and restored-capacity behavior.
• Records Core-LB-G08 evidence in the core load-balancer feature contract.
• Does not start Core-LB-G09.

Tests added/updated:

• Added CoreLoadBalancerOverloadRecoveryScenarioTest.
• Updated docs/agent/CORE_LOADBALANCER_FEATURE_CONTRACT.md with Core-LB-G08 evidence.

Verification:

• mvn test "-Dtest=CoreLoadBalancerOverloadRecoveryScenarioTest" passed: 4 tests, 0 failures, 0 errors, 0 skipped.
• mvn test "-Dtest=CoreLoadBalancerOverloadRecoveryScenarioTest,CoreLoadBalancerServerLifecycleInvariantTest,CoreLoadBalancerCapacityPredictiveOverloadTest,CoreLoadBalancerCrossStrategyEdgeInvariantTest,CoreLoadBalancerDeterministicOrderingTieTest,CoreLoadBalancerLeastLoadedSemanticsTest,CoreRoutingRegistryComparisonContractTest,CoreRoutingDecisionIntegrationTest,LoadBalancerTest,ServerRegistryTest,AgentCoreLoadBalancerFeatureContractDocumentationTest" passed: 126 tests, 0 failures, 0 errors, 0 skipped.
• mvn -q test passed.
• mvn -q "-DskipTests" package passed.
• mvn -B package passed: 2,522 tests, 0 failures, 0 errors, 0 skipped.
• git diff --check passed.
• git diff --cached --check passed.
• git diff --check origin/main...HEAD passed.
• .\scripts\smoke\enterprise-lab-workflow.ps1 -Package passed.

Speculative safety gate:

• Core-LB-G07 PR Core-LB-G07: Add server lifecycle invariant tests #340 was merged first.
• Core-LB-G07 post-merge main CI and CodeQL completed successfully for merge commit 16fc3d3 before this PR was opened.

Safety:

• No production code changed.
• No Maven config changed.
• No CI/workflow changed.
• No Dockerfile/Compose changes.
• No scripts/runtime resources/endpoints/secrets/external targets/deployment/automation changed.
• PR Add next-level architecture skeleton #334 was not touched.

Not-proven boundaries:

• No production readiness/certification.
• No live-cloud or real-tenant validation.
• No benchmark/load/stress proof.
• No throughput/p95/p99 proof.
• No real-world latency improvement claim.
• No replay/export proof.
• No broader automation.