fix: preserve single regex values in converted ServiceMonitors

[immanuwell]

Related to #1219

StringOrArray was marshaling nested single values as JSON arrays, so converted ServiceMonitor objects could send regex: ["..."] instead of regex: "...". That makes VMServiceScrape validation blow up.

This switches MarshalJSON to a value receiver, so converted relabel regex fields stay strings. Tiny fix, low risk.

Repro:

1. Install Prometheus Operator and VictoriaMetrics Operator.
2. Create a ServiceMonitor with single-string regex in metricRelabelings or relabelings.
3. Current master can reject the generated VMServiceScrape with regex ... must be of type string: "array".
4. This branch keeps regex as a JSON string, so the converted object goes through.

Tests:
go test ./internal/controller/operator/factory/converter
cd api && go test ./operator/v1beta1

[cubic-dev-ai]

No issues found across 3 files

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