fix(#2731): skip unauthenticated providers when resolving subagent model

Background subagents (explore/librarian) failed with auth errors because
resolveModelForDelegateTask() always picked the first fallback entry when
availableModels was empty — often an unauthenticated provider like xai
or opencode-go.

Fix: when connectedProvidersCache is populated, iterate fallback chain
and pick the first entry whose provider is in the connected set.
Legacy behavior preserved when cache is null (not yet populated).

- model-selection.ts: use readConnectedProvidersCache to filter fallback chain
- test: 4 new tests for connected-provider-aware resolution

src/tools/delegate-task/model-selection.ts

@@ -2,7 +2,7 @@ import type { FallbackEntry } from "../../shared/model-requirements"
 import { normalizeModel } from "../../shared/model-normalization"
 import { fuzzyMatchModel } from "../../shared/model-availability"
 import { transformModelForProvider } from "../../shared/provider-model-id-transform"
-import { hasConnectedProvidersCache, hasProviderModelsCache } from "../../shared/connected-providers-cache"
+import { hasConnectedProvidersCache, hasProviderModelsCache, readConnectedProvidersCache } from "../../shared/connected-providers-cache"
 import { log } from "../../shared/logger"
 import { parseModelString, parseVariantFromModelID } from "./model-string-parser"
 
@@ -115,11 +115,29 @@ export function resolveModelForDelegateTask(input: {
   const fallbackChain = input.fallbackChain
   if (fallbackChain && fallbackChain.length > 0) {
     if (input.availableModels.size === 0) {
-      const first = fallbackChain[0]
-      const provider = first?.providers?.[0]
-      if (provider) {
-        const transformedModelId = transformModelForProvider(provider, first.model)
-        return { model: `${provider}/${transformedModelId}`, variant: first.variant, fallbackEntry: first, matchedFallback: true }
+      const connectedProviders = readConnectedProvidersCache()
+      if (connectedProviders) {
+        const connectedSet = new Set(connectedProviders)
+        for (const entry of fallbackChain) {
+          for (const provider of entry.providers) {
+            if (connectedSet.has(provider)) {
+              const transformedModelId = transformModelForProvider(provider, entry.model)
+              log("[resolveModelForDelegateTask] fallback chain resolved via connected provider", {
+                provider,
+                model: entry.model,
+              })
+              return { model: `${provider}/${transformedModelId}`, variant: entry.variant, fallbackEntry: entry, matchedFallback: true }
+            }
+          }
+        }
+        log("[resolveModelForDelegateTask] no connected provider found in fallback chain")
+      } else {
+        const first = fallbackChain[0]
+        const provider = first?.providers?.[0]
+        if (provider) {
+          const transformedModelId = transformModelForProvider(provider, first.model)
+          return { model: `${provider}/${transformedModelId}`, variant: first.variant, fallbackEntry: first, matchedFallback: true }
+        }
       }
     } else {
       for (const entry of fallbackChain) {