docs: add @APIResponse annotations to config REST interfaces #550

Summary

Several REST interfaces have @Operation annotations but are missing @APIResponse annotations for HTTP status codes. Adding these improves the generated OpenAPI spec and Swagger UI documentation.

Interfaces to update

Pattern to follow

See IRestAgentStore.java for the established pattern:

@GET
@APIResponse(responseCode = "200", description = "The agent configuration")
@APIResponse(responseCode = "404", description = "Agent not found")
@Operation(description = "Read agent.")
Response readAgent(...);

Common status codes by HTTP method:

• GET by ID → 200, 404
• GET list → 200
• POST create → 201, 400
• PUT update → 200, 400, 404
• DELETE → 204, 404

Check the implementation class when in doubt about which status codes are actually returned.

Acceptance criteria

• All listed methods have @APIResponse annotations
• Status codes match actual behavior
• ./mvnw compile passes