test: add empty active set deadlock and bootstrap tests for audit module

[mateeullahmalik]

Summary

Add unit and system tests that reproduce the testnet deadlock where all supernodes are POSTPONED and the epoch anchor has an empty active set. Without active probers, peer observations cannot be generated, making audit recovery impossible.

Tests Added

Unit tests (x/audit/v1/keeper/enforcement_empty_active_set_test.go)

• TestEnforceEpochEnd_EmptyActiveSet_PostponedCannotRecover — Proves that compliant host-only reports from POSTPONED SNs are insufficient for recovery when no peer observations exist.
• TestEnforceEpochEnd_LegacyRecoveredSN_SurvivesWithReport — Proves that SNs recovered to ACTIVE mid-epoch (via legacy metrics) with audit reports survive the EndBlocker enforcement.

System tests (tests/systemtests/audit_empty_active_set_bootstrap_test.go)

• TestAuditEmptyActiveSetDeadlock_HostOnlyReportsCannotRecover — Full-chain E2E: registers SNs, misses epoch 0, submits host-only reports for 3 epochs — all remain POSTPONED throughout.
• TestAuditEmptyActiveSetBootstrap_LegacyMetricsBreaksDeadlock — Full-chain E2E: same deadlock setup, then legacy metrics recovery breaks the deadlock — SNs survive enforcement and remain ACTIVE.

Context

On testnet (lumera-testnet-2), all supernodes running v2.4.5-testnet were POSTPONED before the audit module activated (legacy staleness handler). The audit module's recovery requires peer observations from active probers, but the active set has been empty since epoch 1. Fix applied in supernode#275.

These tests ensure the edge case is covered for future upgrades.

[roomote-v0]

Rooviewer   See task

Reviewed commit e33eb42. The stabilization changes are solid: replacing client-side target replication with auditQueryAssignedTargets gRPC queries, adding 45s timeout slack for CI, removing race-prone immediate-state assertions, and adding a retry loop for non-deterministic recovery assignment. The MetricsAggregate.Metrics type fix in the deregister test aligns with the proto definition. One minor duplication noted.

• [audit_peer_ports_enforcement_test.go] awaitAtLeastHeightWithSlackPeerPorts is a duplicate of awaitAtLeastHeightWithSlack -- consolidate into shared helpers

Previous reviews

• 7724e68: Review #1
• 3daf7e4: Review #2

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}