test(redis-cloud): add comprehensive test coverage for connectivity, databases, and fixed_databases handlers

[joshrotenberg]

Summary

This PR adds comprehensive test coverage for three critical Redis Cloud API handlers, continuing the testing effort from PR #89.

Test Coverage Added

🔌 Connectivity Handler (19 tests)

• VPC peering operations (create, update, delete, get)
• Private Service Connect (PSC) service operations
• Transit Gateway (TGW) attachment management
• Multi-cloud provider support (AWS, GCP, Azure)
• Comprehensive error handling (401, 403, 404, 500)

💾 Databases Handler (18 tests)

• Full CRUD operations for database management
• Database backup and restore operations
• Database import functionality
• Support for databases with modules (RediSearch, RedisJSON)
• Alert configuration testing
• Clustering and sharding configurations
• Task status tracking with error responses

🔧 Fixed Databases Handler (11 tests)

• Fixed subscription database CRUD operations
• Backup and import operations
• Tag management (create, update, delete)
• Error handling for all failure scenarios

Testing Approach

All tests follow the established patterns from PR #89:

• ✅ Use wiremock for HTTP response mocking
• ✅ Follow OpenAPI specification for response structures
• ✅ Test all handler methods comprehensively
• ✅ Include error handling for common HTTP status codes
• ✅ Use typed request/response structs from the library
• ✅ Support for optional fields and forward compatibility

Test Results

cargo test --package redis-cloud --test connectivity_tests
test result: ok. 19 passed; 0 failed

cargo test --package redis-cloud --test databases_tests  
test result: ok. 18 passed; 0 failed

cargo test --package redis-cloud --test fixed_databases_tests
test result: ok. 11 passed; 0 failed

Related Issues

Continues test coverage improvements started in #89

Checklist

• Tests pass locally
• Code formatted with cargo fmt
• Tests follow existing patterns
• Error cases covered
• Multi-cloud scenarios tested where applicable