[cr149][iOS] Mark CWVWebViewConfiguration.preferences as unavailable

Removes the `NOTREACHED` in `-[CWVWebViewConfiguration preferences]` that is hit in some new `CWVWebView` code, the `NOTREACHED` was originally to avoid developer error in accessing it from Swift, but we can instead just mark it as unavailable with `NS_UNAVAILABLE` to avoid that possibility instead and allow the Obj-C side to just coalesce on null.

Chromium change:

commit 29b0e7808c42609ad6a816161d65acc6d2b46841
Author: Anuj Kumar <anujbh@google.com>
Date:   Fri Apr 17 15:27:16 2026 -0700

    [ios/web_view] Add triggerNonFatalCheck to CWVPreferences

    Adds a new non-persistent, in-memory property 'triggerNonFatalCheck'
    to CWVPreferences. When enabled, it triggers a non-fatal CHECK during
    CWVWebView initialization to facilitate integration testing of
    non-fatal checks by CWV clients.

    Note: All code added in this CL is temporary for release integration
    testing and will be reverted after testing in the stable channel
    concludes.

    The CHECK is implemented as:
    CHECK(false, base::NotFatalUntil::M235); with a distant launch
    milestone to ensure cleanup occurs before it becomes fatal.

    Bug: 503005516
    Change-Id: Ifea699ed37f2760bcc80cc49a71968075c2ad148
    Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7751665
    Reviewed-by: Justin Cohen <justincohen@google.com>
    Commit-Queue: Anuj Kumar <anujbh@google.com>
    Reviewed-by: John Wu <jzw@chromium.org>
    Cr-Commit-Position: refs/heads/main@{#1616907}

chromium_src/ios/web_view/internal/cwv_web_view_configuration.mm

@@ -69,7 +69,6 @@
 - (CWVPreferences*)preferences {
   // This property is not nullable, so will crash anyways on the Swift side if
   // accessed.
-  NOTREACHED();
   return nil;
 }
 

ios/browser/api/web_view/brave_web_view_configuration.h

@@ -11,6 +11,7 @@
 #import "cwv_export.h"                  // NOLINT
 #import "cwv_web_view_configuration.h"  // NOLINT
 
+@class CWVPreferences;
 @class WKWebsiteDataStore;
 @protocol ProfileBridge;
 
@@ -34,6 +35,8 @@ CWV_EXPORT
 + (BraveWebViewConfiguration*)configurationForProfile:
     (id<ProfileBridge>)profileBridge NS_SWIFT_NAME(init(profile:));
 
+@property(nonatomic, readonly) CWVPreferences* preferences NS_UNAVAILABLE;
+
 @end
 
 NS_ASSUME_NONNULL_END

ios/browser/api/web_view/brave_web_view_configuration.mm

@@ -24,6 +24,7 @@
 @implementation BraveWebViewConfiguration {
   CWVAutofillDataManager* _autofillDataManager;
 }
+@dynamic preferences;
 
 - (id<ProfileBridge>)profile {
   auto* profile = ProfileIOS::FromBrowserState(self.browserState);