Deploy main eacf1e7 to prod

Replaces #559, which conflicted.

Model: opus-5-5

internal/config/configtest/configtest.go

@@ -0,0 +1,52 @@
+// Package configtest holds test support for code that reads the
+// process environment.
+package configtest
+
+import (
+	"os"
+	"strings"
+	"testing"
+)
+
+// ClearEnv unsets every variable in the process environment
+// for the rest of the test, so a test sees only the variables it sets
+// itself, not whatever the developer's shell exports. When the test
+// ends it leaves the environment exactly as it found it: each variable
+// it unset is put back, and any variable added since is removed.
+func ClearEnv(t *testing.T) {
+	t.Helper()
+
+	present := make(map[string]bool)
+
+	for _, entry := range os.Environ() {
+		key, _, _ := strings.Cut(entry, "=")
+		present[key] = true
+
+		// t.Setenv registers the restore; the Unsetenv after it is
+		// what makes the key absent, since a key set to the empty
+		// string is still present, and godotenv will not overwrite a
+		// present key.
+		t.Setenv(key, "")
+
+		err := os.Unsetenv(key)
+		if err != nil {
+			t.Fatalf("unsetting %s: %v", key, err)
+		}
+	}
+
+	// A variable the test adds other than through t.Setenv, as loading
+	// a .env file does, has no restore of its own.
+	t.Cleanup(func() {
+		for _, entry := range os.Environ() {
+			key, _, _ := strings.Cut(entry, "=")
+			if present[key] {
+				continue
+			}
+
+			err := os.Unsetenv(key)
+			if err != nil {
+				t.Errorf("unsetting %s: %v", key, err)
+			}
+		}
+	})
+}