test(desktop): fix flaky forum empty-draft replacement e2e (#7981)

## Summary
The `replacement=` cases (`add` and `publish`) of `forum source visit …:
A→B→A retains source draft` in `forum-agent-invitation.spec.ts` are
flaky. They fail at line 449 because the source draft still reads
`@RemoteScout hello`.

Cause: Playwright's `fill("")` on a contenteditable sets a DOM selection
and then presses `Delete`. ProseMirror handles `Delete` with its own
selection, which sometimes hasn't synced from the DOM yet. When that
happens the key press does nothing, `onUpdate` never fires, and the
clear is never recorded. I confirmed this with temporary logging:
failing runs have no authored-content update after the fill. Non-empty
`fill` uses `insertText`, which goes through ProseMirror's DOM input
path, so it isn't affected.

Fix (test only):
- Clear the draft with `ControlOrMeta+a` followed by `Delete`, so
ProseMirror owns the selection.
- Assert the replacement text before navigating away.

### Related issue
None found. The spec hasn't changed since #7125. This test failed in 10
of the last 40 CI runs of `Desktop Smoke E2E (2)`, for example #3700.

### Testing
- `--repeat-each=100 --retries=0 --workers=4` on the two affected cases:
  - Before the fix: 14 of 200 runs failed at `:449`.
  - After the fix: 200 of 200 passed.
- Full `forum-agent-invitation.spec.ts`: 19 passed.
- Pre-push hooks passed (desktop typecheck and desktop tests).

Signed-off-by: Fizz <6be7be23ec5bba868f3e1db5ec86013d76bd1a293dce5b435a192158fec13641@buzz.block.builderlab.xyz>
Co-authored-by: Fizz <6be7be23ec5bba868f3e1db5ec86013d76bd1a293dce5b435a192158fec13641@buzz.block.builderlab.xyz>

desktop/tests/e2e/forum-agent-invitation.spec.ts

@@ -430,8 +430,17 @@ for (const stage of ["add", "publish"] as const) {
         await expect(
           page.getByRole("button", { name: "Remove attachment" }),
         ).toBeVisible();
-      if (replacement !== null)
+      if (replacement === "") {
+        // fill("") presses Delete over a DOM selection ProseMirror may not
+        // have synced yet; select through the editor's own keymap instead.
+        await page.getByTestId("message-input").click();
+        await page.keyboard.press("ControlOrMeta+a");
+        await page.keyboard.press("Delete");
+      } else if (replacement !== null) {
         await page.getByTestId("message-input").fill(replacement);
+      }
+      if (replacement !== null)
+        await expect(page.getByTestId("message-input")).toHaveText(replacement);
       await navigate(1);
       await releaseForumGate(page);
       expect(