fix(agents): scope volcengine-plan/byteplus-plan auth lookup to profile resolution

The configure flow stores auth credentials under `provider: "volcengine"`,
but the coding model uses `volcengine-plan` as its provider. Add a scoped
`normalizeProviderIdForAuth` function used only by `listProfilesForProvider`
so coding-plan variants resolve to their base provider for auth credential
lookup without affecting global provider routing.

Closes #31731

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/agents/auth-profiles/profiles.ts

@@ -1,5 +1,5 @@
 import { normalizeSecretInput } from "../../utils/normalize-secret-input.js";
-import { normalizeProviderId } from "../model-selection.js";
+import { normalizeProviderId, normalizeProviderIdForAuth } from "../model-selection.js";
 import {
   ensureAuthProfileStore,
   saveAuthProfileStore,
@@ -79,9 +79,9 @@ export async function upsertAuthProfileWithLock(params: {
 }
 
 export function listProfilesForProvider(store: AuthProfileStore, provider: string): string[] {
-  const providerKey = normalizeProviderId(provider);
+  const providerKey = normalizeProviderIdForAuth(provider);
   return Object.entries(store.profiles)
-    .filter(([, cred]) => normalizeProviderId(cred.provider) === providerKey)
+    .filter(([, cred]) => normalizeProviderIdForAuth(cred.provider) === providerKey)
     .map(([id]) => id);
 }
 

src/agents/model-selection.test.ts

@@ -8,6 +8,7 @@ import {
   buildModelAliasIndex,
   normalizeModelSelection,
   normalizeProviderId,
+  normalizeProviderIdForAuth,
   modelKey,
   resolveAllowedModelRef,
   resolveConfiguredModelRef,
@@ -64,6 +65,14 @@ describe("model-selection", () => {
     });
   });
 
+  describe("normalizeProviderIdForAuth", () => {
+    it("maps coding-plan variants to base provider for auth lookup", () => {
+      expect(normalizeProviderIdForAuth("volcengine-plan")).toBe("volcengine");
+      expect(normalizeProviderIdForAuth("byteplus-plan")).toBe("byteplus");
+      expect(normalizeProviderIdForAuth("openai")).toBe("openai");
+    });
+  });
+
   describe("parseModelRef", () => {
     it("should parse full model refs", () => {
       expect(parseModelRef("anthropic/claude-3-5-sonnet", "openai")).toEqual({

src/agents/model-selection.ts

@@ -61,6 +61,18 @@ export function normalizeProviderId(provider: string): string {
   return normalized;
 }
 
+/** Normalize provider ID for auth lookup. Coding-plan variants share auth with base. */
+export function normalizeProviderIdForAuth(provider: string): string {
+  const normalized = normalizeProviderId(provider);
+  if (normalized === "volcengine-plan") {
+    return "volcengine";
+  }
+  if (normalized === "byteplus-plan") {
+    return "byteplus";
+  }
+  return normalized;
+}
+
 export function findNormalizedProviderValue<T>(
   entries: Record<string, T> | undefined,
   provider: string,