OpenAPI spec: AbstractActivityPerformanceReport missing activity.experiences[] and report fields

## Summary

The published OpenAPI spec for `AbstractActivityPerformanceReport` and `AbstractActivity` is missing fields that the live API consistently returns.

## Missing fields

### `AbstractActivity.experiences`

The live API returns an `experiences` array inside `activity` on every performance report response, but the spec omits it:

```json
"activity": {
"id": 1018582,
"metrics": [...],
"experiences": [
{ "name": "Experience A", "experienceLocalId": 0 },
{ "name": "Experience B", "experienceLocalId": 1 }
]
}
```

### `AbstractActivityPerformanceReport.report`

The entire `report` block is absent from the spec. This contains all usable statistics:

```json
"report": {
"statistics": {
"experiences": [...],
"totals": { ... }
},
"reportingAudiences": [],
"optionStatistics": []
}
```

## Impact

Without these fields, SDK consumers cannot access experience name/ID mappings or any performance statistics. The `report.statistics.experiences[]` block is the core data the endpoint exists to provide.

## Proposed fix

Add to `AbstractActivity`:
```json
"experiences": {
"type": "array",
"items": { "$ref": "#/components/schemas/ActivityExperience" }
}
```

Add new schemas `ActivityExperience`, `ActivityReportStatistics`, `ActivityReport`, and reference `ActivityReport` from `AbstractActivityPerformanceReport.report`.

## Verification

Verified against the live API on 2026-04-09 using both AB (`/target/activities/ab/{id}/report/performance`) and XT (`/target/activities/xt/{id}/report/performance`) endpoints.

Start by locating the OpenAPI schema definitions for AbstractActivity and AbstractActivityPerformanceReport, then compare them with the AB and XT performance-report endpoints named in the issue. Confirm that ActivityExperience, ActivityReportStatistics, and ActivityReport represent the shown response fields, and verify that experiences and report are referenced in the published spec.

Written by the indexing model from the issue text.