[backend] test(multi-tenancy): dashboards API for multi-tenancy (#5718)

[damgouj]

Proposed changes

• Add tenant isolation and tests for dashboards API

Testing Instructions

1. Check everything is working as before

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality
• For bug fix -> I implemented a test that covers the bug

[Copilot]

Pull request overview

This PR strengthens multi-tenancy isolation for the dashboards feature by tenant-scoping Widgets at the database/model layer and adding integration tests to ensure cross-tenant access is blocked.

Changes:

• Add tenant_id support for widgets (model + Flyway migration) and enforce tenant scoping in lookups.
• Tighten CustomDashboardRepository.findByResourceId() tenant constraints for scenario/exercise-linked dashboards.
• Add tenant isolation integration tests for dashboards, widgets, import/export flows.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
openaev-model/src/main/java/io/openaev/database/repository/WidgetRepository.java	Adds tenant-scoped lookup method for widgets.
openaev-model/src/main/java/io/openaev/database/repository/CustomDashboardRepository.java	Adds tenant constraint to native findByResourceId(); adds tenant-scoped findByIdAndTenantId().
openaev-model/src/main/java/io/openaev/database/model/Widget.java	Converts Widget to TenantBase, adds tenant filter + tenant relationship.
openaev-api/src/main/java/io/openaev/rest/custom_dashboard/WidgetService.java	Uses tenant-scoped dashboard/widget lookups for isolation.
openaev-api/src/main/java/io/openaev/rest/custom_dashboard/CustomDashboardService.java	Uses tenant-scoped dashboard lookup for isolation.
openaev-api/src/main/java/io/openaev/migration/V5_12__AddTenantToWidgets.java	Adds and backfills tenant_id on widgets, adds FK + index.
openaev-api/src/test/java/io/openaev/rest/dashboard/DashboardApiTest.java	Adds tenant isolation tests around custom dashboards + widget dashboard endpoints.
openaev-api/src/test/java/io/openaev/rest/custom_dashboard/CustomDashboardWidgetApiTest.java	Adds tenant isolation tests around widget CRUD on tenant dashboards.
openaev-api/src/test/java/io/openaev/api/custom_dashboard/CustomDashboardApiImporterTest.java	Adds tenant isolation tests for dashboard import.
openaev-api/src/test/java/io/openaev/api/custom_dashboard/CustomDashboardApiExporterTest.java	Adds tenant isolation tests for dashboard export.