Core-LB-G12: Add consistent hashing invariant tests

[RicheyWorks]

Overall Classification: WARN
Campaign: Core LoadBalancer Engineering Campaign
Goal: Core-LB-G12
Scope: test-hardening plus narrow contract documentation for consistent hashing invariants

Summary:

• Adds focused consistent hashing invariant tests for fixed-input deterministic routing.
• Covers invalid key-count rejection and fail-closed empty/all-unhealthy server sets.
• Verifies server removal and duplicate replacement do not leave reviewer-visible stale hash allocations.
• Characterizes zero-load hashing as selected zero-valued buckets.
• Updates the core load-balancer feature contract with G12 evidence.

Tests added/updated:

• Added CoreLoadBalancerConsistentHashingInvariantTest.
• Updated docs/agent/CORE_LOADBALANCER_FEATURE_CONTRACT.md evidence references.

Verification:

• mvn test "-Dtest=CoreLoadBalancerConsistentHashingInvariantTest" passed after correcting a test sequencing assertion: 6 tests, 0 failures, 0 errors, 0 skipped.
• mvn test "-Dtest=CoreLoadBalancerConsistentHashingInvariantTest,ConsistentHashRingTest,LoadBalancerTest,CoreLoadBalancerCrossStrategyEdgeInvariantTest,CoreLoadBalancerDeterministicOrderingTieTest,CoreLoadBalancerServerLifecycleInvariantTest,CoreLoadBalancerOverloadRecoveryScenarioTest,AgentCoreLoadBalancerFeatureContractDocumentationTest,AgentCoreLoadBalancerReviewerEvidenceMapDocumentationTest,AgentCoreLoadBalancerEvidenceConsolidationDocumentationTest" passed: 122 tests, 0 failures, 0 errors, 0 skipped.
• mvn -q test passed.
• mvn -q "-DskipTests" package passed.
• mvn -B package passed: 2,542 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-G11 PR Core-LB-G11: Add weighted distribution invariant tests #344 was merged first.
• Core-LB-G11 post-merge main CI and CodeQL completed successfully for merge commit f70302c 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.
• Core-LB-G13 was not started in this PR.

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.