fix: add OpenAPI spec fixture for CI

The OpenAPI validation tests were failing in CI because the spec file
was in the tmp/ directory which is gitignored.

Changes:
- Moved cloud_openapi.json to crates/redis-cloud/tests/fixtures/
- Updated include_str! path in openapi_validation.rs
- Updated CLAUDE.md to reflect correct spec location

This ensures the OpenAPI validation tests work both locally and in CI.

Author: joshrotenberg

CLAUDE.md

@@ -485,7 +485,7 @@ cargo test --package redis-cloud --test openapi_validation
 ```
 
 ### OpenAPI Spec Location
-- **Local Copy**: `tmp/cloud_openapi.json`
+- **Local Copy**: `crates/redis-cloud/tests/fixtures/cloud_openapi.json`
 - **Official Source**: https://redis.io/docs/latest/operate/rc/api/api-reference/openapi.json
 
 ### API Documentation Pattern