FN-6205: skip custom providers in settings save split

Prevent the settings save split from serializing custom provider definitions through the global or project patch paths.

- skip `customProviders` when building global settings patches
- skip `customProviders` when building project settings patches
- add a regression test that keeps custom providers out of both serialized patches

Files changed:
 .../app/__tests__/settings-save-split.test.ts      | 24 ++++++++++++++++++++++
 .../app/components/settings/save-split.ts          |  4 ++++
 2 files changed, 28 insertions(+)

Fusion-Task-Id: FN-6205

Fusion-Task-Lineage: 84c3f42b-c621-44ff-b977-ecd3a6729e04

packages/dashboard/app/__tests__/settings-save-split.test.ts

@@ -117,6 +117,30 @@ describe("splitSettingsSave", () => {
     expect(projectPatch).toEqual({ enabledBuiltinWorkflowIds: ["builtin:coding"] });
   });
 
+  it("excludes customProviders from both global and project patches", () => {
+    expect(isGlobalSettingsKey("customProviders")).toBe(true);
+
+    const { globalPatch, projectPatch } = splitSettingsSave({
+      payload: {
+        customProviders: [
+          {
+            id: "x",
+            name: "Provider X",
+            baseUrl: "https://example.test/v1",
+            apiKey: "secret",
+            models: [{ id: "model-x", name: "Model X" }],
+          },
+        ],
+      },
+      initialValues: { customProviders: [] } as never,
+      initialScopedValues: { global: { customProviders: [] }, project: {} } as never,
+      activeSection: "authentication",
+    });
+
+    expect("customProviders" in globalPatch).toBe(false);
+    expect("customProviders" in projectPatch).toBe(false);
+  });
+
   it("emits null-as-delete when a project override is cleared", () => {
     const initialScopedValues = {
       global: {},

packages/dashboard/app/components/settings/save-split.ts

@@ -76,6 +76,9 @@ export function splitSettingsSave({
     if (key === "persistAgentThinkingLog") {
       continue;
     }
+    if (key === "customProviders") {
+      continue;
+    }
     if (isGlobalSettingsKey(key)) {
       // null-as-delete: explicit clear is sent as null, plain undefined dropped.
       const initialValue = initialValues?.[key as keyof GlobalSettings];
@@ -90,6 +93,7 @@ export function splitSettingsSave({
   const projectPatch: Partial<Settings> = {};
   for (const [key, value] of Object.entries(payload)) {
     if (key === "githubTokenConfigured" || key === "prAuthAvailable") continue; // server-only
+    if (key === "customProviders") continue;
     if (key === "githubTrackingDefaultRepo" && activeSection === "global-general") continue;
     if (!isProjectSettingsKey(key)) continue;