Fix potential race condition with request ID in dev mode (#84532)
In development mode, the request ID needs to be available before the
bootstrap script executes and triggers hydration. Previously, the
request ID was set inline with the flight data stream in a script tag
that was inserted after the bootstrap script, which could lead to a race
condition where `self.__next_r` was accessed before it was defined.
With this PR, we're moving the request ID initialization to
`bootstrapScriptContent`, which is inserted as a script tag by React
immediately before the bootstrap script. This ensures that the request
ID is always defined when the bootstrap script runs.
I wasn't actually able to reproduce the issue, so I also couldn't add a
test for it. But we did receive reports from users that this was
happening, which is plausible given the previous script order.