# CLOUDP-347497 - Single cluster Replica Set Controller Refactoring

## Why this refactoring

The single-cluster RS controller was mixing two concerns:
- **Kubernetes stuff** (StatefulSets, pods, volumes)
- **Ops Manager/MongoDB stuff** (MongoDB processes, replication config)

This worked fine for single-cluster, but it's a problem when you think
about multi-cluster:
- Multi-cluster has **multiple StatefulSets** (one per cluster) but only
**one logical ReplicaSet** in Ops Manager
- The OM automation config doesn't care about how many K8s clusters you
have or how the pods are deployed

So we need to separate these layers properly.

## Main changes

### 1. Broke down the huge Reconcile() method

Before: ~300 lines of inline logic in Reconcile()

Now:
```go
Reconcile()
  ├── reconcileMemberResources()        // Handles all K8s resource creation
  │   ├── reconcileHostnameOverrideConfigMap()
  │   ├── ensureRoles()
  │   └── reconcileStatefulSet()        // StatefulSet-specific logic isolated here
  │       └── buildStatefulSetOptions() // Builds STS configuration
  └── updateOmDeploymentRs()            // Handles Ops Manager automation config updates
```

This makes it way easier to understand what's happening and matches the
multi-cluster controller structure.

### 2. Removed StatefulSet dependency from OM operations

Created new helper functions that work directly with MongoDB resources
instead of StatefulSets:
- `CreateMongodProcessesFromMongoDB()` - was using StatefulSet before
- `BuildFromMongoDBWithReplicas()` - same
- `WaitForRsAgentsToRegisterByResource()` - same

These mirror the existing `...FromStatefulSet` functions but take
MongoDB resources instead.

**Why it matters:** The OM layer now only cares about the MongoDB
resource definition, not how it's deployed in K8s. This makes the code
work the same way for both single-cluster and multi-cluster.

### 3. Added publishAutomationConfigFirstRS checks

Dedicated function for RS instead of using the shared one. Does not rely
on a statefulset object.
## Important for review

The ideal way to review this PR is to compare the new structure to the
`mongodbmultireplicaset_controller.go`. The aim of the refactoring is to
get the single cluster controller closer to it.

Look at:
- `reconcileMemberResources()` in both controllers - similar structure
now
- `updateOmDeploymentRs()` - no more StatefulSet dependency
- New helper functions in `om/process` and `om/replicaset` - mirror
existing patterns

## Bug found along the way

The logic to handle **scale up + disable TLS at the same time** doesn't
actually work properly and should be blocked by validation (see [draft
PR #490](#490) for
more details).

## Tests added

Added tests for the new helper functions:

- `TestCreateMongodProcessesFromMongoDB` - basic scenarios, scaling,
custom domains, TLS, additional config
- `TestBuildFromMongoDBWithReplicas` - integration test checking
ReplicaSet structure and member options propagation
- `TestPublishAutomationConfigFirstRS` - automation config publish logic
with various TLS/auth scenarios