[6.x] Fix toasts breaking when sessions are serialized as JSON

[mynetx]

Fixes #14797.

Flashing a toast and redirecting, like this:

Toast::success('Success!');

return Inertia::back();

made the next request receive _toasts: [[]] and throw Uncaught TypeError: this[type] is not a function in the console instead of showing the toast.

The toast manager flashes Toast objects into the session. When the response is a redirect, the toast has to survive session persistence into the next request. Laravel supports serializing session data as JSON (session.serialization), and current app skeletons ship 'serialization' => 'json' as the default. Toast only has private properties and didn't implement JsonSerializable, so json_encode() turned it into {}, which decoded back to an empty array on the next request. Flows that deliver toasts in the same response (like saving an entry) never round-trip through the session, which is why they were unaffected.

This makes Toast implement JsonSerializable, so it survives the round-trip. Sessions serialized with the php strategy are unchanged, and the fluent ->duration() chaining after push keeps working since the session still holds the object itself.

Verified in a live CP with JSON session serialization: before, the follow-up request's _toasts prop was [[]]; after, it's [{"message":"Sandbox toast test","type":"success","duration":null}] and the toast displays.

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