Add client behavior fixtures

docs/client-libraries-design.md

@@ -29,6 +29,7 @@ Language-specific plans:
 Shared generation inputs:
 
 - `docs/client-proto-generation.md`
+- `docs/ClientBehaviorFixtures.md`
 - `clients/proto/proto-inputs.json`
 
 Language style guides:
@@ -310,6 +311,11 @@ CLI output should support JSON for automated tests.
 Unit tests must run without a live gateway. Use fake gRPC services, mock
 transports, or generated test servers depending on language.
 
+Shared behavior fixtures live in `clients/proto/fixtures/behavior`. Every
+client should include tests that load the fixture manifest and verify wrapper
+behavior against the common command reply, event stream, value conversion,
+status conversion, auth error, and timeout/cancel cases.
+
 Required unit test areas:
 
 - options parsing,