fix: add spelling allowlist entry and jscpd exclusion for generated code

- Add 'protocgen' to spelling allow.txt (proto2pydantic plugin org)
- Exclude generated *_pydantic.py from jscpd copy/paste detection

Author: brucearctor

.github/actions/spelling/allow.txt

@@ -107,6 +107,7 @@ proto
 protobuf
 Protobuf
 protoc
+protocgen
 pydantic
 pyi
 pypistats

.jscpd.json

@@ -6,7 +6,8 @@
     "**/src/a2a/grpc/**",
     "**/src/a2a/compat/**",
     "**/.nox/**",
-    "**/.venv/**"
+    "**/.venv/**",
+    "**/src/a2a/types/*_pydantic.py"
   ],
   "threshold": 3,
   "reporters": ["html", "markdown"]