encoding/openapi: add regression test for a fixed issue

The issue was opened in early 2022, since the generated OpenAPI
was empty, and that has been fixed as of May 2025.
It's unclear what fixed it, as it could have been the new evaluator.
It doesn't really matter as long as the behavior is correct.

Closes #1643.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Ibf2b412e7ab5aca501ad9ccb873ae6bf7e9771b9
Dispatch-Trailer: {"type":"trybot","CL":1214391,"patchset":1,"ref":"refs/changes/91/1214391/1","targetBranch":"master"}

Author: mvdan

encoding/openapi/testdata/issue1643.txtar

@@ -0,0 +1,54 @@
+-- in.cue --
+#One:{
+	field1: #Id,
+	if true { #Two}
+}
+#Two: { field2: string }
+#Id: 1 | 2
+
+-- out/TestGenerateOpenAPI/out.json --
+{
+   "openapi": "3.0.0",
+   "info": {
+      "title": "Generated by cue.",
+      "version": "no version"
+   },
+   "paths": {},
+   "components": {
+      "schemas": {
+         "Id": {
+            "type": "integer",
+            "enum": [
+               1,
+               2
+            ]
+         },
+         "One": {
+            "type": "object",
+            "required": [
+               "field1",
+               "field2"
+            ],
+            "properties": {
+               "field1": {
+                  "$ref": "#/components/schemas/Id"
+               },
+               "field2": {
+                  "type": "string"
+               }
+            }
+         },
+         "Two": {
+            "type": "object",
+            "required": [
+               "field2"
+            ],
+            "properties": {
+               "field2": {
+                  "type": "string"
+               }
+            }
+         }
+      }
+   }
+}