A startup error was cut at its last "): " to drop the annotations fx
adds, so the blob_size_limit message, which itself contains
"(... size spread): ", lost everything but "need at least N bytes".
cleanStartupError now uses dig.RootCause, from the library fx is built
on, which removes exactly those annotations and returns the error
underneath unchanged.
The test starts a real fx app on a config whose blob_size_limit is below
the minimum. The old cases fed it plain strings that only looked like fx
output; such an error is now rightly left whole, so they are replaced.
go.uber.org/dig, already required through fx, becomes a direct
requirement in go.mod.
Model: opus-5-5
sneak