fix: stop unified-mailbox from mutating client-returned email objects

fetchUnifiedEmails, fanOutUnifiedQuery and the cross-account fanOutCrossQuery
stamped accountId/accountLabel/source* directly onto each email object
returned by the per-account client. Those objects are shared references;
mutating them in place could surprise any caller that retained them (and
corrupt an account-state snapshot). Decorate shallow copies instead, at all
three fan-out sites.

The original fix/unified-mailbox-no-mutation branch predated the cross-account
"All accounts" feature and only covered two sites; this re-applies the fix to
main's current code, including the third (shared/group) fan-out site, and
preserves all five stamped fields. Flips the characterisation test to assert
the client's object is left untouched.

lib/__tests__/unified-mailbox.test.ts

@@ -120,16 +120,17 @@ describe('fetchUnifiedEmails', () => {
     expect(result).toEqual({ emails: [], total: 0, hasMore: false, errors: new Map() });
   });
 
-  it('CHARACTERISATION: mutates the source email objects in place (shared reference)', async () => {
+  it('does NOT mutate the source email objects (decorates copies)', async () => {
     const original = makeEmail('m1', '2026-01-01T00:00:00Z');
     const acc = makeAccount(
       { accountId: 'A', accountLabel: 'Label A', mailboxes: [makeMailbox({ role: 'inbox' })] },
       { getEmails: vi.fn(async (): Promise<FetchResult> => ({ emails: [original], total: 1, hasMore: false })) },
     );
-    await fetchUnifiedEmails([acc], 'inbox', 20, 0);
+    const res = await fetchUnifiedEmails([acc], 'inbox', 20, 0);
-    // The very object passed back by the client was mutated, not a copy.
+    // The returned email carries the account info, but the client's object is untouched.
-    expect(original.accountId).toBe('A');
+    expect(res.emails[0]).toMatchObject({ id: 'm1', accountId: 'A', accountLabel: 'Label A' });
-    expect(original.accountLabel).toBe('Label A');
+    expect('accountId' in original).toBe(false);
+    expect('accountLabel' in original).toBe(false);
   });
 });
 

lib/unified-mailbox.ts

@@ -119,16 +119,19 @@ export async function fetchUnifiedEmails(
 
     const { account, result } = outcome.value;
 
-    // Decorate each email with the source account info.
+    // Decorate each email with the source account info. The per-account client
-    for (const email of result.emails) {
+    // returns shared object references; decorate shallow copies instead of
-      email.accountId = account.accountId;
+    // mutating them in place so retained callers/snapshots aren't corrupted.
-      email.accountLabel = account.accountLabel;
+    const decorated = result.emails.map((email) => ({
-      email.sourceClientAccountId = account.clientAccountId;
+      ...email,
-      email.sourceAccountId = account.jmapAccountId;
+      accountId: account.accountId,
-      email.sourceFolder = resolveSourceFolderName(email, account.mailboxes);
+      accountLabel: account.accountLabel,
-    }
+      sourceClientAccountId: account.clientAccountId,
+      sourceAccountId: account.jmapAccountId,
+      sourceFolder: resolveSourceFolderName(email, account.mailboxes),
+    }));
 
-    mergedEmails = mergedEmails.concat(result.emails);
+    mergedEmails = mergedEmails.concat(decorated);
     totalSum += result.total;
     if (result.hasMore) {
       anyHasMore = true;
@@ -247,14 +250,16 @@ async function fanOutUnifiedQuery(
   for (const outcome of results) {
     if (outcome.status !== 'fulfilled' || outcome.value === null) continue;
     const { account, result } = outcome.value;
-    for (const email of result.emails) {
+    // Decorate shallow copies, not the shared client-returned objects.
-      email.accountId = account.accountId;
+    const decorated = result.emails.map((email) => ({
-      email.accountLabel = account.accountLabel;
+      ...email,
-      email.sourceClientAccountId = account.clientAccountId;
+      accountId: account.accountId,
-      email.sourceAccountId = account.jmapAccountId;
+      accountLabel: account.accountLabel,
-      email.sourceFolder = resolveSourceFolderName(email, account.mailboxes);
+      sourceClientAccountId: account.clientAccountId,
-    }
+      sourceAccountId: account.jmapAccountId,
-    mergedEmails = mergedEmails.concat(result.emails);
+      sourceFolder: resolveSourceFolderName(email, account.mailboxes),
+    }));
+    mergedEmails = mergedEmails.concat(decorated);
     totalSum += result.total;
     if (result.hasMore) anyHasMore = true;
   }
@@ -383,14 +388,16 @@ async function fanOutCrossQuery(
   for (const outcome of results) {
     if (outcome.status !== 'fulfilled' || outcome.value === null) continue;
     const { account, result } = outcome.value;
-    for (const email of result.emails) {
+    // Decorate shallow copies, not the shared client-returned objects.
-      email.accountId = account.accountId;
+    const decorated = result.emails.map((email) => ({
-      email.accountLabel = account.accountLabel;
+      ...email,
-      email.sourceClientAccountId = account.clientAccountId;
+      accountId: account.accountId,
-      email.sourceAccountId = account.jmapAccountId;
+      accountLabel: account.accountLabel,
-      email.sourceFolder = resolveSourceFolderName(email, account.mailboxes);
+      sourceClientAccountId: account.clientAccountId,
-    }
+      sourceAccountId: account.jmapAccountId,
-    mergedEmails = mergedEmails.concat(result.emails);
+      sourceFolder: resolveSourceFolderName(email, account.mailboxes),
+    }));
+    mergedEmails = mergedEmails.concat(decorated);
     totalSum += result.total;
     if (result.hasMore) anyHasMore = true;
   }