fix: Observable properties passed from an element to another without an initializer are not responsive in Safari 26

[albertor24]

🐛 Bug Report
In Safari 26, observable properties passed from one FASTElement to another do not update reactively under certain conditions.

💻 Repro or Code Sample
https://stackblitz.com/edit/typescript-7vs2owmc?file=index.ts

🤔 Expected Behavior
Both prop1 and prop2 should update reactively in my-element-two when their corresponding values in my-element change.

😯 Current Behavior
In Safari 26, prop1 updates as expected, but prop2 (anotherProp) does not reflect changes reactively in my-element-two.

💁 Possible Solution
This may be related to Safari’s handling of property bindings or shadow DOM updates. We have seen that properties marked from boundObservables

fast/packages/web-components/fast-element/src/components/element-controller.ts

Line 259 in 15f32c3

delete element[propertyName];

do work, and all others don't

🔦 Context
This issue affects component composition and data flow in FASTElement-based applications when targeting Safari users. It limits the reliability of reactive bindings between components and for our app; it requires us to manually initialize every property passed by a parent

🌍 Your Environment

OS & Device: macOS 26.0 on MacMini
Browser: Apple Safari
Version: Safari 26