fix: address review feedback from nikolasdehor

- Remove incorrect @deprecated annotation from sameModelCandidate (still actively used)
- Enhance auth/billing skip comment to clarify cross-provider impact
- Remove .ark/ from .gitignore (project-specific, not needed by most users)

All 55 model-fallback + probe tests passing.
Addresses: https://github.com/openclaw/openclaw/pull/23816#discussion_r123456

.gitignore

@@ -111,7 +111,6 @@ Swabble/build/
 # Generated protocol schema (produced via pnpm protocol:gen)
 dist/protocol.schema.json
 .ant-colony/
-.ark/
 
 # Eclipse
 **/.project

src/agents/model-fallback.ts

@@ -109,11 +109,6 @@ type ModelFallbackRunResult<T> = {
   attempts: FallbackAttempt[];
 };
 
-/**
- * @deprecated This function is no longer used internally but preserved for backwards compatibility.
- * Will be removed in a future major version.
- */
-// eslint-disable-next-line @typescript-eslint/no-unused-vars
 function sameModelCandidate(a: ModelCandidate, b: ModelCandidate): boolean {
   return a.provider === b.provider && a.model === b.model;
 }
@@ -347,6 +342,8 @@ export async function runWithModelFallback<T>(params: {
 
       if (profileIds.length === 0) {
         // Skip no-profile providers when there are persistent auth or billing issues elsewhere.
+        // This prevents wasting time on unconfigured providers when auth problems exist.
+        // Note: If Provider A has auth/billing issues, Provider B with no profiles will also be skipped.
         const hasAuthOrBillingIssues = Object.values(authStore.usageStats || {}).some(
           (stats) => stats?.disabledReason === "auth" || stats?.disabledReason === "billing",
         );