test(controlplane): make invalid contract fixture fail on value not unknown field

With unknown-field tolerance (DiscardUnknown) in contract parsing, a contract
whose only problem was an unknown field is now valid. Update the invalid YAML
fixture to fail validation for a real reason (invalid material name) so
TestIdentifyAndValidateRawContract keeps asserting a validation error.

Assisted-by: Claude Code
Signed-off-by: Jose I. Paris <jiparis@chainloop.dev>

Chainloop-Trace-Sessions: ec69dde4-acef-48c8-b319-76774e77c877

Author: jiparis

app/controlplane/pkg/biz/testdata/contracts/invalid_contract.yaml

@@ -1,2 +1,4 @@
 schemaVersion: v1
-notAContract: true
+materials:
+  - type: CONTAINER_IMAGE
+    name: INVALID_NAME