[macOS] Fix FlutterTextInputPluginTest crash - #194581

Fix was generated by gemini:

In FlutterTextInputPluginTest.mm, TextInputTestViewController previously
overrode `-createFlutterViewWithMTLDevice:commandQueue:` to return
`OCMClassMock([NSView class])`, which allocates a 72-byte
`OCClassMockObject` (`NSProxy` subclass) rather than a real `NSView`.


https://github.com/flutter/flutter/blob/449fb191435ed8ae509b71623142fc35e6932f46/engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterTextInputPluginTest.mm#L49-L52

When `-[FlutterViewController loadView]` added that mock to a real
NSView (`FlutterViewWrapper`) via `-[NSView addSubview:]`, AppKit
directly accessed `NSView` instance variables at offsets beyond the
72-byte `OCClassMockObject` allocation, corrupting adjacent blocks in
the malloc freelist and triggering a libsystem_malloc EXC_BREAKPOINT
(memory corruption of free block) on subsequent allocations.


https://github.com/flutter/flutter/blob/449fb191435ed8ae509b71623142fc35e6932f46/engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterViewController.mm#L384-L400

To fix this, `TextInputTestViewController` now overrides `-loadView` to
set `self.view` to a real `NSView` instance and `-flutterView` to return
the `OCMClassMock([FlutterView class])` directly without adding the
`NSProxy` mock to the AppKit view hierarchy.

---

Tested and verified manually:

I was able to re-create the crash locally and therefore verify it was
fixed using:
`./engine/src/out/host_release_arm64/flutter_desktop_darwin_unittests
--gtest_shuffle --gtest_repeat=10`

Fixes flutter#194251.

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