fix: exclude signature from signing string in BoxIMClient

Prevents stale signature values from leaking into the MD5 signing
calculation, matching the news_service.py pattern. Adds a test that
passes a stale signature in extra params and verifies the returned
signature is freshly computed.

digital-avatar-app/backend/tests/test_boxim_client.py

@@ -98,6 +98,28 @@ def test_build_sign_params_contains_required_fields(mock_config):
     assert len(params["nonce"]) == 12
 
 
+def test_build_sign_params_excludes_signature_and_accessSecret_from_signing_string(mock_config):
+    """signature and accessSecret must be excluded from the signing string to match news_service.py."""
+    from services.boxim_client import BoxIMClient
+
+    client = BoxIMClient(mock_config)
+
+    # Pass params that already contain a stale "signature" value
+    params_with_stale_sig = client._build_sign_params({
+        "userId": "u1",
+        "signature": "OLD_STALE_SIG",
+    })
+
+    # The returned signature must be freshly computed (32-char MD5 uppercase),
+    # NOT the stale value we passed in.
+    assert params_with_stale_sig["signature"] != "OLD_STALE_SIG"
+    assert len(params_with_stale_sig["signature"]) == 32
+
+    # Calling with the same extra params but no stale signature should also work.
+    params_clean = client._build_sign_params({"userId": "u1"})
+    assert len(params_clean["signature"]) == 32
+
+
 def test_build_sign_params_signature_is_deterministic(mock_config):
     """Same inputs should produce valid MD5 signatures."""
     from services.boxim_client import BoxIMClient