[Flight] Define Flight chunk `.then` with `Object.defineProperty` (#37109)

[Secure Ecmascript](https://github.com/tc39/proposal-ses) would freeze
the prototype of intrinsics. Since `ReactPromise` inherits the prototype
from `Promise`, it also copies over the writable definition.

Using `defineProperty` on an inherited property is compatible with SES
though. That's also closer to how classes are specced in JS.


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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>