[lexical-history] Bug Fix: SharedHistoryExtension forwards the parent maxDepth

[LeSingh1]

Description

SharedHistoryExtension redirects a nested editor's history signals at the
parent's, so both editors share one undo stack. It forwards four of the five
writable signals:

batch(() => {
  output.delay.value = parentOutput.delay.value;
  output.historyState.value = parentOutput.historyState.value;
  output.now.value = parentOutput.now.value;
  // Note that toggling the parent history will force this to be changed
  output.disabled.value = parentOutput.disabled.value;
});                                   // maxDepth is never copied

maxDepth is a config-derived, writable Signal<number | null> on
HistoryExtensionOutput exactly like the others, and it caps the stack in
applyChange. The child's registerHistory therefore runs with the parent's
historyState but the child's own maxDepth, which defaults to null —
no cap.

So every history event originating in the nested editor pushes onto the shared
undoStack without applying the limit the application configured on the
parent. An app that sets maxDepth to bound memory silently loses that bound
for anything typed in a nested editor (the playground's sticky notes use
SharedHistoryExtension).

The HistoryExtensionOutput docstring has the same omission — it enumerates
delay, disabled, historyState and now as the signals
SharedHistoryExtension redirects. maxDepth was added later and was missed
in both places, so this updates the doc alongside the code.

Test plan

New unit test packages/lexical-history/src/__tests__/unit/SharedHistoryMaxDepth.test.ts.
Deliberately a new file: PR #8953 (also mine) edits the HistoryExtension maxDepth block in LexicalHistory.test.tsx, and keeping these apart avoids a
self-conflict. The second case asserts the four signals that already forward —
it passes before and after, which is what pins the gap to maxDepth alone.

Before

$ npx vitest run packages/lexical-history/src/__tests__/unit/SharedHistoryMaxDepth.test.ts

     × the child adopts the parent maxDepth 7ms

AssertionError: expected null to be 7 // Object.is equality

      Tests  1 failed | 1 passed (2)

After

$ npx vitest run packages/lexical-history/src/__tests__/unit/SharedHistoryMaxDepth.test.ts

      Tests  2 passed (2)

[vercel]

@LeSingh1 is attempting to deploy a commit to the Meta Open Source Team on Vercel.

A member of the Team first needs to authorize it.

[mayrang]

Confirmed the bug on main (null vs 7) and 27/27 tests pass on this branch. The fix is correct and matches the pattern of the four sibling signals. Two small notes inline.