fix(environment): answer a missing entity with a 2xx null only for reads

A served write of a missing entity also looked at the write tool's own
recordings, so a delete that recorded 200 null for a successful
deletion answered a missing target with success. A 2xx null is now an
absence only when the call is a read, as the gate takes it from item
reads; a write still answers with a recorded 400, 404 or 410, or is
refused.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/application/environment/environmentStateAnswer.ts

@@ -264,7 +264,9 @@ export const missingAnswer = (
     readonly id: string;
   },
 ): Result<StateAnswer, AnalysisError> => {
-  // The write gate takes a 2xx JSON null as an absence too, so a delete it confirmed that way can be answered.
+  // A read also answers with a 2xx JSON null, the absence the write gate accepts; a write never does, as a 2xx would
+  // report success for an entity that is not there.
+  const nullAbsence = tool.method === "GET";
   const latestAbsence = (ordinals: readonly number[]) =>
     ordinals
       .map((ordinal) => serving.source.exchanges.get(ordinal))
@@ -272,7 +274,8 @@ export const missingAnswer = (
         (exchange): exchange is RecordedExchange =>
           exchange !== undefined &&
           (isAbsence(exchange.status) ||
-            (exchange.status !== null &&
+            (nullAbsence &&
+              exchange.status !== null &&
               exchange.status >= 200 &&
               exchange.status < 300 &&
               recordedJson(exchange) === null)),

src/application/environment/environmentStateWriteAnswer.test.ts

@@ -472,4 +472,20 @@ describe("missing entities after a delete", () => {
       state: { view: "missing", id: "1" },
     });
   });
+
+  it("never answers a write of a missing entity with success, even when the write recorded a 2xx null", () => {
+    const books = servedFrom([
+      ...BOOK_INIT_ROWS,
+      ["DELETE", "/api/books/2", null, 200],
+      ["GET", "/api/books/2", BOOK_NOT_FOUND, 404],
+    ]);
+    expect(books.state.writes.get("delete_books_by_id")).toMatchObject({
+      served: true,
+    });
+    expect(books.answer("delete_books_by_id", { id: "9" })).toMatchObject({
+      status: 404,
+      body: BOOK_NOT_FOUND,
+      state: { view: "missing", id: "9" },
+    });
+  });
 });