build: use fixed a2a.proto commit hash (#762)

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Author: bartek-w

buf.gen.yaml

@@ -2,7 +2,7 @@
 version: v2
 inputs:
   - git_repo: https://github.com/a2aproject/A2A.git
-    ref: main
+    ref: 1997c9d63058ca0b89361a7d6e508f4641a6f68b
     subdir: specification
 managed:
   enabled: true