test(ui): pin deriveErrorMessage against the ProxyException wire shape

Both automated reviewers assumed the semantic filter 400 reaches the
browser as FastAPI's flat detail dict and would render as raw JSON in
the test panel banner. The proxy converts a pre-call hook HTTPException
into a ProxyException that serializes as {"error": {"message": ...}},
which deriveErrorMessage unpacks first; pin that contract with direct
tests

ui/litellm-dashboard/src/lib/http/client.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect, vi } from "vitest";
-import { createApiClient, ApiError } from "./client";
+import { createApiClient, ApiError, deriveErrorMessage } from "./client";
 
 const okResponse = (data: unknown): Response =>
   ({ ok: true, status: 200, text: async () => JSON.stringify(data) }) as unknown as Response;
@@ -101,3 +101,22 @@ describe("createApiClient", () => {
     }
   });
 });
+
+describe("deriveErrorMessage", () => {
+  it("extracts error.message from a ProxyException body, the shape the proxy emits for a pre-call hook HTTPException", () => {
+    const actionable =
+      "MCP semantic tool filtering could not run: embedding model 'text-embedding-3-small' exceeded its context window while embedding the user query. The request was blocked instead of silently passing all tools through. Switch to an embedding model with a larger context window, or disable semantic tool filtering.";
+    const wireBody = {
+      error: { message: actionable, type: "None", param: "None", code: "400" },
+    };
+    expect(deriveErrorMessage(wireBody)).toBe(actionable);
+  });
+
+  it("returns error directly when it is a plain string", () => {
+    expect(deriveErrorMessage({ error: "flat error text" })).toBe("flat error text");
+  });
+
+  it("falls back to a string detail field", () => {
+    expect(deriveErrorMessage({ detail: "detail text" })).toBe("detail text");
+  });
+});