Add explicit build tags to platform-specific echo test files

The Go toolchain infers constraints from _darwin/_linux filename suffixes,
but explicit //go:build tags make the constraint visible without relying
on filename conventions, consistent with modern Go style.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/prompter/echo_darwin_test.go

@@ -1,3 +1,5 @@
+//go:build darwin
+
 package prompter_test
 
 import "golang.org/x/sys/unix"

internal/prompter/echo_linux_test.go

@@ -1,3 +1,5 @@
+//go:build linux
+
 package prompter_test
 
 import "golang.org/x/sys/unix"