Implement transaction support for batch requests with POST operations

[kbarbounakis]

Background
The batch feature in OData v4 allows for complex workflows involving multiple operations within a single request. However, when multiple POST requests (inserts) are included in a batch, it's crucial to ensure that either all operations succeed or the batch is rolled back if any fail, maintaining data integrity.

Task

• Design and implement transaction handling for batch requests containing POST operations.
• Ensure that if any POST operation in a batch fails, all changes from that batch are rolled back to prevent partial insertions.
• Support atomicityGroup/changeset semantics according to OData v4 standards.
• Handle errors correctly (e.g., 424 Failed Dependency when dependencies exist and a parent operation fails).
• Provide appropriate feedback in the batch response for transaction outcomes.

Acceptance Criteria

• POST requests in a batch are executed in an atomic transaction group.
• On failure of one or more POSTs, all requests in the group are rolled back.
• Tests are provided for success, failure, and rollback scenarios.
• The feature is documented (examples, API usage, edge-cases involving nested objects or arrays).

References

• Main tracking issue: Improve batch documentation: Content-ID references, $$ syntax, and atomicityGroup (changeset) support #87
• OData v4 Batch Protocol and atomicityGroup docs.