Keep a grading-heartbeat fault from replacing the grade

[ppXD]

Summary

• A grading-heartbeat pulse that failed ended its loop faulted, and all three grade call sites in SupervisorTurnService.Rehydrate.cs (resolve fold, per-unit fold, stop-target grade) await that loop in the finally around the grade, so the pulse's error replaced the grade. The live trigger: the pulse's IRunRecordLogger shared the turn service's scoped CodeSpaceDbContext, so a pulse landing mid-query died on EF's "a second operation was started on this context" guard. The loop is now HeartbeatLoop.RunAsync (a failed pulse is a structured warning naming {SupervisorRunId}/{NodeId}, and the next pulse still fires), and each pulse resolves its record logger from its own DI scope via IServiceScopeFactory, the one new constructor parameter. The call-site finally shape is unchanged; its catch (OperationCanceledException) was already unreachable and stays as a guard.
• The heartbeat tests pinned how many beats landed, not when: main's HeartbeatLoopTests pass a loop that sleeps half its interval. Each beat is now bracketed (a step short of the interval: nothing; the step that completes it: exactly one), and SupervisorLane.AcceptanceGradeHeartbeatInterval, documented as pinned but never pinned, is pinned at 90s inside the reconciler's liveness window.
• HonestNoContinuityHint now ends "(your prior attempt pushed no branch of its own for this repository)", which stays true for a multi-repo attempt whose primary push failed but a sibling's succeeded (AgentCodeNode.RepinWorkspaceToPriorAttempt); nothing in the frontend reads the text. LoopbackModelCredentialBroker.AcceptAsync now says why an accept wait lost to a close strands nothing: the stranded loop and its lease are an unrooted cycle the GC collects (checked on .NET 10; no code change). Two stale comments are fixed (SupervisorDependencyStagingTests, ArtifactRetentionPolicy.For).

Test plan

• Integration SupervisorGradingHeartbeatIsolationFlowTests: a real query holds the grade scope's context while a pulse falls due. Red on main (the pulse's InvalidOperationException surfaced from the grade's finally); green now (the pulse row lands on its own context and the grade survives)
• Unit SupervisorGradingHeartbeatTests (6) and HeartbeatLoopTests (4); stable over 5 integration and 10 unit repeats
• Mutations: swallow-and-continue removed → A_failed_pulse_is_reported_and_never_replaces_the_grade red; per-pulse scope removed → the integration test red (no pulse lands within 30s) plus 4 unit tests; one scope per loop → 2 unit tests red; grading loop sleeps interval / 2 → cadence test red; AcceptanceGradeHeartbeatInterval halved → pin red; HeartbeatLoop sleeps interval / 2 → 3 tests red (main's HeartbeatLoopTests stay 4/4 green)
• Full unit suite: 10949 passed, 1 skipped (pre-existing)
• Integration, every touched class plus the retry world-state and agent-node classes that read the hint: 233 run, 232 passed, 1 real-model skip
• dotnet build: 0 errors, no new warnings