solver-api: standardize error contracts and harden parsing paths

Add structured 4xx responses and defensive parser/process behavior so clients receive consistent failures and malformed inputs are rejected deterministically.

Made-with: Cursor

solver-api/tests/api.test.js

@@ -78,6 +78,23 @@ describe("solver-api", () => {
     expect(Array.isArray(response.body.parsed.unsupportedFields)).toBe(true);
   });
 
+  it("rejects /solve without xml and returns schema metadata", async () => {
+    const app = buildApp();
+    const response = await request(app).post("/solve").send({});
+    expect(response.status).toBe(400);
+    expect(response.body.schemaVersion).toBe(2);
+    expect(response.body.code).toBe("missing_xml");
+  });
+
+  it("rejects diagnostic workflow without surface card", async () => {
+    const app = buildApp();
+    const response = await request(app)
+      .post("/solve")
+      .send({ xml, solverModel: "fdm", workflow: "diagnostic" });
+    expect(response.status).toBe(400);
+    expect(response.body.code).toBe("missing_surface_card");
+  });
+
   it("is deterministic for the same input", async () => {
     const app = buildApp();
     const a = await request(app).post("/solve").send({ xml });