Step-by-step guide for converting Spring Boot JPA applications to use Azure Cosmos DB with Spring Data Cosmos

# Convert Spring JPA project to Spring Data Cosmos

This generalized guide applies to any JPA to Spring Data Cosmos DB conversion project.

## High-level plan

1. Swap build dependencies (remove JPA, add Cosmos + Identity).
2. Add `cosmos` profile and properties.
3. Add Cosmos config with proper Azure identity authentication.
4. Transform entities (ids → `String`, add `@Container` and `@PartitionKey`, remove JPA mappings, adjust relationships).
5. Convert repositories (`JpaRepository` → `CosmosRepository`).
6. **Create service layer** for relationship management and template compatibility.
7. **CRITICAL**: Update ALL test files to work with String IDs and Cosmos repositories.
8. Seed data via `CommandLineRunner`.
9. **CRITICAL**: Test runtime functionality and fix template compatibility issues.

## Step-by-step

### Step 1 — Build dependencies

- **Maven** (`pom.xml`):
  - Remove dependency `spring-boot-starter-data-jpa`
  - Remove database-specific dependencies (H2, MySQL, PostgreSQL) unless needed elsewhere
  - Add `com.azure:azure-spring-data-cosmos:5.17.0` (or latest compatible version)
  - Add `com.azure:azure-identity:1.15.4` (required for DefaultAzureCredential)