`--state` option outputs `metadata`

From help,

    --state                     Only output diff state: no_changes,
                                incompatible, compatible

this doesn't align with reality. We had a case where the state outputted metadata.

Caused by this line:
https://github.com/OpenAPITools/openapi-diff/blob/1de574d3ca447bc463740bc10c37851702974a00/cli/src/main/java/org/openapitools/openapidiff/cli/Main.java#L203

We can simply
https://github.com/OpenAPITools/openapi-diff/blob/1de574d3ca447bc463740bc10c37851702974a00/core/src/main/java/org/openapitools/openapidiff/core/model/DiffResult.java#L5

to include something like

public DiffResult toStateDiff() {
  if (this.isUnchanged()) {
    return DiffResult.UNCHANGED;
  }

 if (this.isCompatible()) {
    return DiffResult.COMPATIBLE;
 }

 return DiffResult.INCOMPATIBLE;
}

Start in cli/src/main/java/org/openapitools/openapidiff/cli/Main.java at the --state handling, then read core/src/main/java/org/openapitools/openapidiff/core/model/DiffResult.java and its state values. Confirm that --state reports only no_changes, compatible, or incompatible, and verify that a metadata result is no longer emitted.