Retrieves comprehensive details for a single CRM activity identified by its unique Seismic activity identifier. This endpoint returns the same detailed information structure as the list endpoint but for a specific activity record. Use this endpoint when you need to investigate a particular activity identified from the list endpoint results, retrieve full details after receiving an activity ID from another source or notification, debug individual activity processing issues by examining error details, audit a specific CRM synchronization event for compliance, or build drill-down interfaces that navigate from activity lists to detailed views. The response includes complete activity metadata, processing status, timestamps, CRM context, and any error information if processing failed. Example scenario: After calling GET /v1/activities and identifying activities with status='Failed', an agent can call this endpoint with each failed activityId to retrieve detailed error messages and determine the root cause. Another example: When a user clicks on an activity in a dashboard, retrieve full details using this endpoint to display all available information including external system references and processing history.