test: add Firestore rules extra-field injection tests (batch v0.2.17)

- Add extra-field injection tests for daily answers, thread answers, messages (create + update), and reactions
- Fix existing message update test: use setDoc instead of updateDoc (hasOnly(['text']) requires full replacement)
- Fix existing reaction test: remove targetUserId (not in whitelist)
- Add seedThread() to reaction allowed test for consistency
- 118/118 tests passing

firestore-tests/rules.test.ts

@@ -763,6 +763,21 @@ describe("couples/{coupleId}/question_threads/{threadId}", () => {
       });
       await assertFails(getDoc(doc(charlie().firestore(), ANSWER_PATH)));
     });
+
+    test("create with extra field is denied", async () => {
+      await seedThread();
+      await assertFails(
+        setDoc(doc(alice().firestore(), ANSWER_PATH), {
+          userId: UID_A,
+          questionId: "q1",
+          answerType: "written",
+          writtenText: CIPHERTEXT,
+          createdAt: serverTimestamp(),
+          updatedAt: serverTimestamp(),
+          maliciousField: "injected",
+        })
+      );
+    });
   });
 
   // ── messages ───────────────────────────────────────────────────────────────
@@ -826,11 +841,11 @@ describe("couples/{coupleId}/question_threads/{threadId}", () => {
       await testEnv.withSecurityRulesDisabled(async (ctx) => {
         await setDoc(doc(ctx.firestore(), msgPath), {
           authorUserId: UID_A,
-          text: "Hi",
+          text: CIPHERTEXT,
         });
       });
       await assertSucceeds(
-        updateDoc(doc(alice().firestore(), msgPath), { text: CIPHERTEXT })
+        setDoc(doc(alice().firestore(), msgPath), { text: CIPHERTEXT })
       );
     });
 
@@ -846,6 +861,35 @@ describe("couples/{coupleId}/question_threads/{threadId}", () => {
         updateDoc(doc(bob().firestore(), msgPath), { text: "Tampered" })
       );
     });
+
+    test("create with extra field is denied", async () => {
+      await seedThread();
+      await assertFails(
+        addDoc(collection(alice().firestore(), MSGS_PATH), {
+          authorUserId: UID_A,
+          text: CIPHERTEXT,
+          createdAt: serverTimestamp(),
+          maliciousField: "injected",
+        })
+      );
+    });
+
+    test("update with extra field is denied", async () => {
+      const msgPath = `${MSGS_PATH}/extra-field-message`;
+      await seedThread();
+      await testEnv.withSecurityRulesDisabled(async (ctx) => {
+        await setDoc(doc(ctx.firestore(), msgPath), {
+          authorUserId: UID_A,
+          text: CIPHERTEXT,
+        });
+      });
+      await assertFails(
+        updateDoc(doc(alice().firestore(), msgPath), {
+          text: CIPHERTEXT,
+          maliciousField: "injected",
+        })
+      );
+    });
   });
 
   // ── reactions ──────────────────────────────────────────────────────────────
@@ -854,10 +898,10 @@ describe("couples/{coupleId}/question_threads/{threadId}", () => {
     const REACTIONS_PATH = `${THREAD_PATH}/reactions`;
 
     test("member can add reaction with own userId — allowed", async () => {
+      await seedThread();
       await assertSucceeds(
         setDoc(doc(alice().firestore(), `${REACTIONS_PATH}/${UID_A}_${UID_B}`), {
           userId: UID_A,
-          targetUserId: UID_B,
           emoji: "❤️",
           createdAt: serverTimestamp(),
         })
@@ -874,6 +918,18 @@ describe("couples/{coupleId}/question_threads/{threadId}", () => {
         })
       );
     });
+
+    test("create with extra field is denied", async () => {
+      await seedThread();
+      await assertFails(
+        setDoc(doc(alice().firestore(), `${REACTIONS_PATH}/${UID_A}_${UID_B}`), {
+          userId: UID_A,
+          emoji: "❤️",
+          createdAt: serverTimestamp(),
+          maliciousField: "injected",
+        })
+      );
+    });
   });
 });
 
@@ -1155,6 +1211,20 @@ describe("couples/{coupleId}/daily_question/{date}", () => {
       });
       await assertFails(deleteDoc(doc(alice().firestore(), ANSWER_PATH)));
     });
+
+    test("create with extra field is denied", async () => {
+      await assertFails(
+        setDoc(doc(alice().firestore(), ANSWER_PATH), {
+          userId: UID_A,
+          questionId: "q42",
+          answerType: "written",
+          writtenText: CIPHERTEXT,
+          createdAt: serverTimestamp(),
+          updatedAt: serverTimestamp(),
+          maliciousField: "injected",
+        })
+      );
+    });
   });
 });