Extend GET /v1/analytics/sessions with bounce/exit metrics and variant dimension

Description

Extend the GET /v1/analytics/sessions endpoint in CAEM to support bounce and exit session metrics, a variant dimension for grouping, and new optional query parameters for experiment-based filtering.

New metrics:

• Bounce sessions count
• Bounce rate
• Exit sessions count
• Exit rate

New dimension:

• Variant — enables grouping session data by experiment variant

New query parameters:

• experimentId — filter sessions by experiment
• runningId — filter by running experiment instance
• referencePage — reference page used to calculate exit rate

The new metrics query against the session-level fact table, separate from the existing engagement aggregation path. Includes a new repository component, service routing logic, and integration tests.

Acceptance Criteria

• GET /v1/analytics/sessions returns bounce sessions count when the bounce sessions metric is requested
• GET /v1/analytics/sessions returns bounce rate when the bounce rate metric is requested
• GET /v1/analytics/sessions returns exit sessions count when the exit sessions metric is requested
• GET /v1/analytics/sessions returns exit rate when the exit rate metric is requested
• Session data can be grouped by the variant dimension
• The endpoint accepts and applies experimentId, runningId, and referencePage as optional filter parameters
• Existing session metrics and endpoint behavior are not regressed
• Integration tests pass for bounce rate calculations
• Integration tests pass for exit rate calculations
• Postman tests covering the new metrics and parameters are added to the CAEM Postman collection

Additional Context

Implementation repository: dotCMS/dot-ca-event-manager

Start in the linked dotCMS/dot-ca-event-manager repository at the GET /v1/analytics/sessions endpoint and trace the existing session metrics aggregation and service routing. Run the integration tests and inspect the CAEM Postman collection; done means the new metrics, variant grouping, filters, regression coverage, and Postman tests all pass.