Add rate limiting and input validation security enhancements

[smahima27]

Summary

Implements API security enhancements

• Rate limiting middleware (Redis-backed)
• Input validation for all critical endpoints
• Protection against abuse and injection attacks

Changes

• Rate Limiter: 100 req/min per IP, 500 req/min authenticated, 20/min VM creation, 50/min VM deletion
• Input Validator: Hostname, pool name, tag, VM count, disk size, lifetime, JSON body validation
• API Security: 6 critical endpoints secured with validation (POST /vm, POST /vm/:template, GET/PUT/DELETE /vm/:hostname, POST /vm/:hostname/disk/:size)

Testing

• ✅ 27 unit tests for input validation (all passing)
• ✅ Full integration test suite: 849 examples, 0 failures
• ✅ Code coverage: 83.71%
• ✅ All RuboCop offenses fixed

Jira

P4DEVOPS-9434