fix(commandboard-api): return 400 for malformed JSON bodies instead of 500

Malformed JSON on any POST endpoint previously bubbled a SyntaxError to
the top-level handler, which mapped every throw to 500. Client input
errors now respond 400 { "error": "Invalid JSON body" } via a typed
InvalidJsonBodyError thrown from readJson().

Adds contract tests covering all four POST endpoints plus a control
asserting schema-invalid (but well-formed) bodies still return 422.

Fixes #11

apps/commandboard-api/src/contract.test.ts

@@ -182,3 +182,34 @@ describe("CommandBoard API contracts", () => {
     expect(Array.isArray(body.errors)).toBe(true);
   });
 });
+
+describe("invalid JSON request bodies", () => {
+  const postEndpoints = [
+    "/api/tasks",
+    "/api/plugins/sh1pt/actions/publish",
+    "/api/plugins/c0mpute/jobs/dispatch",
+    "/api/plugins/c0mpute/quotes"
+  ];
+
+  it.each(postEndpoints)("returns 400 (not 500) for malformed JSON on %s", async (endpoint) => {
+    const response = await fetch(`${baseUrl}${endpoint}`, {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: "{invalid json"
+    });
+    const body = await response.json() as { error: string };
+
+    expect(response.status).toBe(400);
+    expect(body).toEqual({ error: "Invalid JSON body" });
+  });
+
+  it("still returns 422 for well-formed JSON that fails schema validation", async () => {
+    const response = await fetch(`${baseUrl}/api/tasks`, {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: JSON.stringify({ title: "missing required fields" })
+    });
+
+    expect(response.status).toBe(422);
+  });
+});

apps/commandboard-api/src/index.ts

@@ -60,6 +60,10 @@ export function createCommandBoardServer() {
     try {
       await route(request, response);
     } catch (error) {
+      if (error instanceof InvalidJsonBodyError) {
+        json(response, 400, { error: error.message });
+        return;
+      }
       json(response, 500, { error: error instanceof Error ? error.message : String(error) });
     }
   });
@@ -281,13 +285,24 @@ function text(response: ServerResponse, status: number, contentType: string, bod
   response.end(body);
 }
 
+class InvalidJsonBodyError extends Error {
+  constructor() {
+    super("Invalid JSON body");
+    this.name = "InvalidJsonBodyError";
+  }
+}
+
 async function readJson(request: IncomingMessage) {
   const chunks: Buffer[] = [];
   for await (const chunk of request) {
     chunks.push(Buffer.from(chunk));
   }
 
-  return JSON.parse(Buffer.concat(chunks).toString("utf8")) as unknown;
+  try {
+    return JSON.parse(Buffer.concat(chunks).toString("utf8")) as unknown;
+  } catch {
+    throw new InvalidJsonBodyError();
+  }
 }
 
 function isRecord(value: unknown): value is Record<string, unknown> {