PromQL: support or with on/ignoring modifiers

## What

`or` with matching modifiers is rejected by the verifier:

```
set operator [or] with on/ignoring is not supported at this time
```

In Prometheus, `on(...)`/`ignoring(...)` on `or` changes the dedup identity: a right-hand series is dropped when the left operand contains any series matching on the reduced key set, rather than on the full label set. The current UNION translation dedups on the full label identity only.

Rejection site: `PromqlCommand.verifySetOperator`.