Add unit tests for all 10 patterns

[prashantpiyush1111]

Feature Request

Problem

Currently all patterns are tested manually via browser or curl commands.
No automated unit tests exist for any pattern.

Patterns That Need Tests

• 01 - Circuit Breaker
• 02 - Rate Limiting
• 03 - Retry Logic
• 04 - Idempotency
• 05 - Caching
• 06 - Bulkhead
• 07 - Health Check
• 08 - Throttling
• 09 - Graceful Shutdown
• 10 - Dead Letter Queue

Expected

Each pattern should have JUnit 5 tests covering:

• Happy path (success scenario)
• Failure scenario (circuit open, rate limit exceeded, etc.)
• Edge cases

Tech

• JUnit 5
• Mockito
• Spring Boot Test (@SpringBootTest)