FN-6012: quarantine flaky dashboard chat manager test

Quarantine the flaky dashboard chat manager test and keep it out of active API lanes.

- record packages/dashboard/src/__tests__/chat-manager.test.ts in the quarantine ledger with the current failure context and quarantine date
- exclude the quarantined chat-manager test from dashboard API backfill coverage so existing missions surface an explicit no-backfill decision
- exclude the quarantined chat-manager test from the broad dashboard API project so its status stays visible without destabilizing the suite

Files changed:
 packages/dashboard/vitest.config.ts | 3 ++-
 scripts/lib/test-quarantine.json    | 8 +++++++-
 2 files changed, 9 insertions(+), 2 deletions(-)

Fusion-Task-Id: FN-6012

Fusion-Task-Lineage: 34014c45-486e-443d-ae47-a1d301a396e1

packages/dashboard/vitest.config.ts

@@ -258,6 +258,7 @@ const qualityAppBackfillTests = ["app/**/*.test.{ts,tsx}"];
 const backfillApiExclude = [
   ...qualityApiTests,
   ...skipListDashboardGlobs.filter((file) => file.startsWith("src/")),
+  "src/__tests__/chat-manager.test.ts",
 ];
 const qualityApiBackfillTests = ["src/**/*.test.{ts,tsx}"];
 
@@ -449,7 +450,7 @@ export default defineConfig({
           name: "dashboard-api",
           environment: "node",
           include: ["src/**/*.test.{ts,tsx}"],
-          exclude: [],
+          exclude: ["src/__tests__/chat-manager.test.ts"],
           css: { include: [] },
         },
       },