[pull] main from react:main - #625
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Added behind a new experimental flag, `enableFlightWeakThenables`. Adds a new thenable status to the Flight protocol: `'pending_weak'`. Unlike a regular pending thenable, a weak thenable does not block the stream from closing. If it settles while the stream is still open, its value is emitted like a normal pending thenable. Otherwise its reference is left unfulfilled and on the client it stays forever pending, without erroring, even when the connection closes. It's up to the client to handle the unresolved promise in an appropriate way. The motivating use case is being able to encode metadata about a Flight stream into the response itself. For example, a framework might want to track whether a page varies by search params. It could represent this in the response as a `Promise<boolean>` that resolves to `true` as soon as the component being rendered in the stream accesses search params. If the thenable never resolves by the time the stream closes, then the client knows that no search params were ever accessed. In the future we could add a higher-level API for encoding this kind of information. For now, we intentionally start with the low-level primitive so frameworks can experiment in userspace without adding significantly to React's surface area. Internally, Flight already uses its own private thenable statuses, like `'resolved_model'`, and the protocol is designed to treat any status besides `'fulfilled'` and `'rejected'` as equivalent to `'pending'`, so `'pending_weak'` slots into the existing machinery. On the wire, a weak reference is encoded as `$w<id>`, next to `$@<id>` for regular promises, so the client knows its row may intentionally never arrive. On the client, a weak reference behaves like any other pending promise until the response closes; then, instead of erroring, it is left forever pending.
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