Keep the whole startup error when it contains "): " (closes #320)

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

go.mod

@@ -22,6 +22,7 @@ require (
 	github.com/spf13/cobra v1.10.1
 	github.com/spf13/pflag v1.0.10
 	github.com/stretchr/testify v1.11.1
+	go.uber.org/dig v1.19.0
 	go.uber.org/fx v1.24.0
 	golang.org/x/sync v0.18.0
 	golang.org/x/sys v0.38.0
@@ -256,7 +257,6 @@ require (
 	go.opentelemetry.io/otel/metric v1.38.0 // indirect
 	go.opentelemetry.io/otel/trace v1.38.0 // indirect
 	go.shabbyrobe.org/gocovmerge v0.0.0-20230507111327-fa4f82cfbf4d // indirect
-	go.uber.org/dig v1.19.0 // indirect
 	go.uber.org/multierr v1.11.0 // indirect
 	go.uber.org/zap v1.27.0 // indirect
 	go.yaml.in/yaml/v2 v2.4.3 // indirect