Prior to this commit, ExchangeResult.assertWithDiagnostics() threw an
IllegalArgumentException for a custom HTTP status code since toString()
invoked getStatus() without a try-catch block.
This commit addresses this issue by introducing a formatStatus() method
that defensively formats the response status, initially trying to
format the HttpStatus and falling back to formatting the raw integer
status code.
Closes gh-29283
b20de758b2