Allow uppercase letters in secret names (closes #2)

The isValidSecretName() regex only allowed lowercase letters [a-z], rejecting
valid secret names containing uppercase characters (e.g. AWS access key IDs).

Changed regex from ^[a-z0-9\.\-\_\/]+$ to ^[a-zA-Z0-9\.\-\_\/]+$ and added
tests for uppercase secret names in both vault and secret packages.

internal/vault/secrets.go

@@ -67,7 +67,7 @@ func (v *Vault) ListSecrets() ([]string, error) {
 	return secrets, nil
 }
 
-// isValidSecretName validates secret names according to the format [a-z0-9\.\-\_\/]+
+// isValidSecretName validates secret names according to the format [a-zA-Z0-9\.\-\_\/]+
 // but with additional restrictions:
 // - No leading or trailing slashes
 // - No double slashes
@@ -93,7 +93,7 @@ func isValidSecretName(name string) bool {
 	}
 
 	// Check the basic pattern
-	matched, _ := regexp.MatchString(`^[a-z0-9\.\-\_\/]+$`, name)
+	matched, _ := regexp.MatchString(`^[a-zA-Z0-9\.\-\_\/]+$`, name)
 
 	return matched
 }