fix(server): return validation error bodies

packages/opencode/src/server/routes/instance/httpapi/middleware/schema-error.ts

@@ -0,0 +1,21 @@
+import { Effect } from "effect"
+import { HttpServerResponse } from "effect/unstable/http"
+import { HttpApiMiddleware } from "effect/unstable/httpapi"
+
+export class SchemaErrorMiddleware extends HttpApiMiddleware.Service<SchemaErrorMiddleware>()(
+  "@opencode/ExperimentalHttpApiSchemaError",
+) {}
+
+export const schemaErrorLayer = HttpApiMiddleware.layerSchemaErrorTransform(SchemaErrorMiddleware, (error) => {
+  if (error.kind === "Body") return Effect.fail(error)
+  return Effect.succeed(
+    HttpServerResponse.jsonUnsafe(
+      {
+        data: {},
+        errors: [{ kind: error.kind, message: error.cause.message }],
+        success: false,
+      },
+      { status: 400 },
+    ),
+  )
+})