Whether a resource resolves to a known length or not is more than just
a property of the resource & data source - for example if
`DataSpec.flags` contains `ALLOW_GZIP` then the length might be
unresolved. More generally, a `DataSource` could randomly return
`C.UNKNOWN_LENGTH` from `open()` 50% of the time and still fulfil the
`DataSource` interface. This makes it ~impossible to write a meaningful
assertion aroun this.
So this change relaxes the assertion slightly to more closely match the
definition of the `DataSource` interface.
We leave the `resolveToUnknownLength` toggle in
`WebServerDispatcher.Resource` because this is still useful for
simulating the case of a server that is serving a file it doesn't
know the length of.
PiperOrigin-RevId: 351124246
