build: pin A2A spec to avoid breaking ci (#803)

# Description

This PR a2aproject/A2A#1507 introduces a
breaking change in the specification which is going to be addressed
separately.

Author: guglielmo-san

.github/actions/spelling/excludes.txt

@@ -10,6 +10,7 @@
 (?:^|/)pyproject.toml
 (?:^|/)requirements(?:-dev|-doc|-test|)\.txt$
 (?:^|/)vendor/
+(?:^|/)buf.gen.yaml
 /CODEOWNERS$
 \.a$
 \.ai$
@@ -91,3 +92,4 @@ CHANGELOG.md
 ^tests/
 .pre-commit-config.yaml
 (?:^|/)a2a\.json$
+

buf.gen.yaml

@@ -2,7 +2,7 @@
 version: v2
 inputs:
   - git_repo: https://github.com/a2aproject/A2A.git
-    ref: main
+    ref: aca981cee3e7a3f22a4df8fb8a5302406f7a1cf5
     subdir: specification
 managed:
   enabled: true