fix(tools): add key type validation and tighten test assertions in sign_firmware

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

tools/test_sign_firmware.py

@@ -1,7 +1,8 @@
-import os, sys, tempfile
+import sys
 from pathlib import Path
 
 import pytest
+from cryptography.exceptions import InvalidSignature
 from cryptography.hazmat.primitives.asymmetric import ec
 from cryptography.hazmat.primitives import hashes, serialization
 from cryptography.hazmat.primitives.asymmetric.utils import decode_dss_signature
@@ -59,5 +60,5 @@ def test_wrong_key_fails_verification(keypair, tmp_path):
     from cryptography.hazmat.primitives.asymmetric.utils import encode_dss_signature
     sig_der = encode_dss_signature(r, s)
 
-    with pytest.raises(Exception):
+    with pytest.raises(InvalidSignature):
         other_key.public_key().verify(sig_der, b"firmware", ec.ECDSA(hashes.SHA256()))