Improved Unfocus support for Picker on Mac Catalyst (#33127)

# PR Changes - MacCatalyst Picker Improvements

Fixes #30897
Fixes #30891

## Overview
Improved MacCatalyst Picker implementation with proper focus management

## Changes Made

### 1. **PickerHandler.iOS.cs** - MacCatalyst-specific improvements

#### Added UIAlertController instance field
- Declared `UIAlertController? pickerController` as an instance field to
enable proper dismissal via `Unfocus` command
- Allows external code to programmatically close the picker dialog

#### Improved picker dismiss logic
- Moved picker dismiss logic from event handler to the "Done" button
action
- Removed `EditingDidEnd` event handler that was causing duplicate
dismiss calls
- Simplified focus state management by handling it directly in the Done
action

#### Added MapUnfocus command handler
- Implemented `MapUnfocus` method for MacCatalyst to programmatically
dismiss the picker
- Properly updates `IsFocused` and `IsOpen` states when dismissed
- Includes null checks for safety

### 2. **PickerHandler.cs** - Command mapper registration

#### Registered Unfocus command for MacCatalyst
- Added `#elif MACCATALYST` section to CommandMapper
- Registered `nameof(IPicker.Unfocus)` command to enable programmatic
picker dismissal on MacCatalyst
- Brings MacCatalyst picker behavior in line with Android implementation

## Testing
Issue2339

## Platforms Affected
- **MacCatalyst** (primary)
- iOS (no behavior changes, shared code)

## Breaking Changes
None - this is purely an implementation improvement with no API changes.

Author: kubaflo