fix(operator): Only deploy an online store when it is declared

[nikolauspschuetz]

Closes #6586.

Problem

The Feast operator always deployed an online store service (including a serving pod), even when the FeatureStore CR did not declare spec.services.onlineStore. This makes it impossible to run a standalone registry or offline store (e.g. the documented federated / shared-registry pattern), and is inconsistent with how registry and offlineStore behave — both are only defaulted when declared.

Cause

In ApplyDefaultsToStatus (infra/feast-operator/internal/controller/services/util.go), the online-store block force-created the service when nil:

if services.OnlineStore == nil {
    services.OnlineStore = &feastdevv1.OnlineStore{}
}
// ...defaults always applied...

whereas registry and offlineStore are gated behind if services.X != nil.

Fix

Gate the online-store defaulting behind services.OnlineStore != nil, mirroring the two sibling services. A declared online store still receives all of its defaults (persistence, path, PVC, server, container configs).

Tests

Added util_test.go with two specs for ApplyDefaultsToStatus:

• a CR without an online store → Status.Applied.Services.OnlineStore stays nil (fails on master, passes with this change),
• a CR that declares an online store → it still receives its defaults.

Verified with KUBEBUILDER_ASSETS=... go test ./internal/controller/services/ (envtest 1.31.0); go build ./..., go vet, and gofmt are clean. The existing controller tests are unaffected because they declare onlineStore explicitly.

[nikolauspschuetz]

The red `unit-test-python (3.11, macos-14)` leg is an unrelated CI flake, not caused by this change (which is Go-only, scoped to `infra/feast-operator/`):

```
ERROR sdk/python/tests/unit/test_feature_server.py::test_push_and_get
AssertionError: stdout: b''
stderr: b"Command timed out after 120s: [\x27apply\x27]"
2259 passed, 47 skipped, 1 error
```

A `feast apply` subprocess timed out after 120s on the macOS-14 runner. The macOS-3.12 leg and all Ubuntu legs (3.10/3.11/3.12) passed on this same commit, and `operator-test` is green. I do not have permission to re-run the job from the fork — a maintainer re-run of that single leg should clear it.