Follow-up to https://github.com/elastic/kibana/pull/181501
This moves the alert details context used by the AI Assistant from the
apm api `GET /internal/apm/assistant/get_obs_alert_details_context` to
the Obs api
`/internal/observability/assistant/alert_details_contextual_insights`.
This also involves adding a type safe API client to the observability
API tests
### Other changes:
- Rename `synthtraceEsClient` to `apmSynthtraceEsClient` to indicate it
is specific to APM
