…lutter#176329)

## Description

This PR fixes a crash in the macOS text input plugin that occurs when
`NSAttributedString` is passed to the `insertText:replacementRange:`
method instead of `NSString`.

### Root Cause

The `insertText:replacementRange:` method of `NSTextInputClient`
protocol can receive either `NSString` or `NSAttributedString`. The
previous implementation directly called `UTF8String` on the input
parameter without type checking. Since `NSAttributedString` doesn't
respond to the `UTF8String` selector, this caused a crash with the
error:

-[NSConcreteMutableAttributedString UTF8String]: unrecognized selector
sent to instance

### Changes Made
- Added type checking to determine if the input is `NSAttributedString`
- Extract the underlying `NSString` using `[string string]` before
calling `UTF8String`
- This follows the same pattern already implemented in the
`setMarkedText:selectedRange:replacementRange:` method in the same file

### Crash Stack Trace

Thread 0 Crashed: 0 CoreFoundation __exceptionPreprocess 1
libobjc.A.dylib objc_exception_throw 2 CoreFoundation
-[NSObject(NSObject) doesNotRecognizeSelector:] 3 FlutterMacOS
-[FlutterTextInputPlugin insertText:replacementRange:]
(FlutterTextInputPlugin.mm:784) 4 AppKit
-[NSTextInputContext(NSInputContext_WithCompletion)
insertText:replacementRange:completionHandler:]

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- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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---------

Co-authored-by: Kim Ki Cheol <[email protected]>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>