fix: guard focus/keyboard callbacks with mounted checks in QuillRawEditorState

[hardiklakhalani]

Description

Fixes a lifecycle race where _handleFocusChanged schedules post-frame work that can run after QuillRawEditorState is disposed, causing a crash when createKeyboardAppearance() accesses context.

Problem

When the render object is dirty, _handleFocusChanged:

1. Calls requestKeyboard() immediately
2. Schedules another _handleFocusChanged() via addPostFrameCallback
   If the editor is unmounted between scheduling and execution (e.g. parent widget structure changes during async UI updates), the callback throws:

Related Issues

Related #2737

Type of Change

• ✨ Feature: New functionality without breaking existing features.
• 🛠️ Bug fix: Resolves an issue without altering current behavior.
• 🧹 Refactor: Code reorganization, no behavior change.
• ❌ Breaking: Alters existing functionality and requires updates.
• 🧪 Tests: New or modified tests
• 📝 Documentation: Updates or additions to documentation.
• 🗑️ Chore: Routine tasks, or maintenance.
• ✅ Build configuration change: Build/configuration changes.