API Builder V1: Contract mapper for snake_case API boundary

[nicdavidson]

Goal
Centralize payload translation so UI internals can stay camelCase while API requests always match backend validation.

Scope

• Create mapper layer for API payloads: base_path
• Create mapper layer for endpoint payloads: api_id, is_active, request_schema, response_schema, execution_plan, response_mapping
• Apply to create+update paths
• Normalize backend responses back into UI model

Acceptance Criteria

• No 400 validation failures due to key naming mismatch
• Create/update API and endpoint paths succeed via same mapper

Candidate files

• src/app/adf-api-builder/services/api-builder-mapper.service.ts
• src/app/adf-api-builder/df-api-builder-page.component.ts

[nicdavidson]

Delivered as part of API Builder V1 (adf-api-builder module). Closing tracking issue.