fix(windows): drop Dart replies that outlive their webview

[palmoni5]

The crash

Closing a plugin tab in Otzaria while an Otzaria.call() round trip was still pending killed the process:

Faulting module: flutter_inappwebview_windows_plugin.dll
Exception code: 0xc0000005 (access violation)

flutter_inappwebview_plugin::UserContentController::createContentWorld +0x56
  at windows/in_app_webview/user_content_controller.cpp:303

Line 303 dereferences webView_ on a UserContentController that the log shows was already destroyed:

[WindowsInAppWebViewWidget] ... calling "dispose" using []
... dealloc CustomPlatformView
... dealloc InAppWebView
... dealloc UserContentController
[sentry] INFO entering signal handler      <-- the pending reply arrives here

Root cause

InAppWebView::onCallJsHandler builds its reply handlers as [this, callHandlerID] and calls evaluateJavascript on this when Dart answers. Nothing keeps the webview alive across that round trip, so a reply that arrives after the webview is gone runs on freed memory. The same pattern is used by every other callback in in_app_webview.cpp (shouldOverrideUrlLoading, shouldInterceptRequest, download and auth callbacks and so on) — the deferral they hold keeps the WebView2 event alive, not the plugin object.

The fix

• InAppWebView owns a lifetime token (aliveToken_) that expires first thing in its destructor.
• BaseCallbackResult takes an optional owner weak reference and turns every handler (success, error, notImplemented) into a no-op once that owner is gone, logging the drop in debug builds. Callbacks that set no owner behave exactly as before.
• All 15 callback sites in in_app_webview.cpp pass the token.

Verification

• ctest on the example's native test target: 11/11 pass, including 4 new BaseCallbackResult cases covering owner-alive, owner-gone and the no-owner default.
• Otzaria debug build against this branch, closing a plugin tab repeatedly mid-request: the process stays up and the log shows the reply being dropped right after teardown, where it used to crash.

... dealloc InAppWebView
... dealloc UserContentController
base_callback_result.h(34): dropping a Dart reply that outlived its owner

The test runner now links flutter and flutter_wrapper_plugin, which base_callback_result.h needs.