fix(agent-service): handle falsy model throws

agent-service/src/agent/texera-agent.spec.ts

@@ -670,6 +670,25 @@ describe("sendMessage", () => {
     expect(agent.getAllSteps()[1].content).toBe("Error: just-a-string");
   });
 
+  test.each([null, undefined, false, 0, ""])("a falsy throw resolves as an error step: %p", async thrown => {
+    const model = new MockLanguageModelV4({
+      doGenerate: async () => {
+        throw thrown;
+      },
+    });
+    const agent = makeAgentWith(model);
+    const res = await agent.sendMessage("hi");
+    const expected = String(thrown);
+    expect(res).toEqual({
+      response: "",
+      messages: [],
+      usage: { inputTokens: 0, outputTokens: 0, totalTokens: 0 },
+      stopped: false,
+      error: expected,
+    });
+    expect(agent.getAllSteps()[1].content).toBe(`Error: ${expected}`);
+  });
+
   test("a failed turn stays on the branch", async () => {
     const model = new MockLanguageModelV4({
       doGenerate: async () => {

agent-service/src/agent/texera-agent.ts

@@ -657,7 +657,8 @@ export class TexeraAgent {
         stopped: false,
       };
     } catch (error: any) {
-      const isAborted = error.name === "AbortError" || this.abortController?.signal.aborted;
+      const errorMessage = error?.message || String(error);
+      const isAborted = error?.name === "AbortError" || this.abortController?.signal.aborted;
 
       if (isAborted) {
         stepIndex++;
@@ -693,7 +694,7 @@ export class TexeraAgent {
         stepId: stepIndex,
         timestamp: Date.now(),
         role: "agent",
-        content: `Error: ${error.message || String(error)}`,
+        content: `Error: ${errorMessage}`,
         isBegin: false,
         isEnd: true,
       };
@@ -705,7 +706,7 @@ export class TexeraAgent {
         messages: [],
         usage: { inputTokens: 0, outputTokens: 0, totalTokens: 0 },
         stopped: false,
-        error: error.message || String(error),
+        error: errorMessage,
       };
     } finally {
       this.abortController = null;