[1/n] [Serve] Refactor replica rank to prepare for node local ranks

[abrarsheikh]

1. Removed RAY_SERVE_FAIL_ON_RANK_ERROR feature flag - Rank errors now always fail instead of conditional behavior

2. Extracted generic RankManager class - Created reusable rank management logic separated from deployment-specific concerns

3. Introduced ReplicaRank schema - Type-safe rank representation replacing raw integers

4. Simplified error handling - Removed conditional error paths and always enforce rank system invariants

5. Updated tests - Refactored unit tests to use new API and removed flag-dependent test cases

Impact:

• Cleaner separation of concerns in rank management
• More robust error handling with consistent behavior
• Foundation for future multi-level rank support

Next PR #58473