tests: Fix subgraph-data-sources integration test

Replace hardcoded IPFS hash with `@source-subgraph@` placeholder that
gets patched at runtime with the actual deployment hash.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: lutter

tests/integration-tests/subgraph-data-sources/subgraph.yaml

@@ -6,7 +6,7 @@ dataSources:
     name: Contract
     network: test
     source:
-      address: 'QmRWTEejPDDwALaquFGm6X2GBbbh5osYDXwCRRkoZ6KQhb'
+      address: '@source-subgraph@'
       startBlock: 0
     mapping:
       apiVersion: 0.0.7