fix(control-plane): guard non-object JSON in approval-body parsing

CodeRabbit follow-up review on #672 (the only thread that didn't auto-
resolve after the previous fix pushes). The issue technically lives in
#671's plans.handler.ts but CodeRabbit only flagged it now after seeing
the post-fix state.

`JSON.parse("null")` returns null, `JSON.parse("[1,2]")` returns an
array, `JSON.parse("42")` returns a number. All three are syntactically
valid JSON but none of them is the object payload approvePlan/rejectPlan
expect. The previous code would parse successfully then crash later
when dereferencing `body.implementationModel` on `null` (TypeError) or
silently accept arrays and primitives as if they were valid bodies.

Reject these early with HTTP 400 (`code: "invalid_body"`) via the
existing InvalidApprovalBodyError path. Two new regression cases cover
JSON-null and JSON-array bodies.

Verification: npm run typecheck && npm test -w @open-inspect/control-plane
(65/65 files, 2 new test cases green).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bleleve

packages/control-plane/src/session/http/handlers/plans.handler.test.ts

@@ -224,6 +224,36 @@ describe("plansHandler.approvePlan / rejectPlan body validation", () => {
     expect(planService.approvePlanAndFlush).not.toHaveBeenCalled();
   });
 
+  it("approvePlan returns 400 when body is JSON null (not an object)", async () => {
+    // Regression test for CodeRabbit #672 follow-up: JSON.parse("null")
+    // succeeds with value null; the previous code then crashed later when
+    // dereferencing body.implementationModel. The guard rejects non-object
+    // payloads early.
+    const { handler, planService } = createHandler();
+    const req = new Request("http://internal/internal/plan/approve", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: "null",
+    });
+    const res = await handler.approvePlan(req);
+    expect(res.status).toBe(400);
+    const body = (await res.json()) as { code?: string };
+    expect(body.code).toBe("invalid_body");
+    expect(planService.approvePlanAndFlush).not.toHaveBeenCalled();
+  });
+
+  it("approvePlan returns 400 when body is a JSON array", async () => {
+    const { handler, planService } = createHandler();
+    const req = new Request("http://internal/internal/plan/approve", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: "[1,2,3]",
+    });
+    const res = await handler.approvePlan(req);
+    expect(res.status).toBe(400);
+    expect(planService.approvePlanAndFlush).not.toHaveBeenCalled();
+  });
+
   it("approvePlan forwards explicit null reasoning effort to clear the value", async () => {
     const { handler, planService } = createHandler();
     vi.mocked(planService.approvePlanAndFlush).mockResolvedValue({

packages/control-plane/src/session/http/handlers/plans.handler.ts

@@ -183,12 +183,26 @@ async function readApprovalBody(request: Request, log: Logger): Promise<Approval
   if (request.headers.get("content-length") === "0") return {};
   const text = await request.text();
   if (!text.trim()) return {};
+  let parsed: unknown;
   try {
-    return JSON.parse(text) as ApprovalRequestBody;
+    parsed = JSON.parse(text);
   } catch (e) {
     log.warn("plans.approval.invalid_body", { error: e instanceof Error ? e : String(e) });
     throw new InvalidApprovalBodyError(e);
   }
+  // JSON.parse("null") returns null, JSON.parse("[1]") returns an array,
+  // and JSON.parse("42") returns a number — all syntactically valid JSON
+  // but not the object payload we expect. Treat them as malformed bodies
+  // so callers don't dereference properties on non-objects (which would
+  // either crash with TypeError on null, or silently no-op on primitives
+  // and let arrays through as objects).
+  if (!parsed || typeof parsed !== "object" || Array.isArray(parsed)) {
+    log.warn("plans.approval.invalid_body", {
+      error: `Expected JSON object, got ${parsed === null ? "null" : Array.isArray(parsed) ? "array" : typeof parsed}`,
+    });
+    throw new InvalidApprovalBodyError(new Error("Expected JSON object"));
+  }
+  return parsed as ApprovalRequestBody;
 }
 
 function errorResponseForApproval(e: unknown, log: Logger, logEvent: string): Response {