feat: Standardize webhook payload

[youbek]

Summary

Task-specific fields are no longer merged at the top level in webhook POSTs. Webhooks now deliver a flat protocol envelope and place task data under result.

Motivation

• Align webhook structure with the adcp/client behavior, which already expects task data to be nested under a result field.
• Eliminate ambiguity and field collisions in webhook payloads.
• Make webhook envelope consistent with other protocol wrappers.
• Provide a stable, protocol-focused top-level with a single result container.

What changed

• Schema: static/schemas/v1/core/webhook-payload.json
  • Added: operation_id (required), timestamp? (ISO 8601)
  • Kept: context_id?, task_id?, task_type (required), status (required), message?
  • Added: result? (any), error? (string)
  • Removed: domain, created_at, updated_at, completed_at, progress
  • Updated examples and notes to nest task data under result
• Registry: static/schemas/v1/index.json
  • Clarified description: protocol fields top-level, task payload under result
• Docs (examples and guidance updated to use result):
  • docs/protocols/task-management.mdx
  • docs/protocols/core-concepts.mdx
  • docs/protocols/envelope-examples.mdx
  • docs/media-buy/task-reference/create_media_buy.mdx
  • docs/media-buy/task-reference/update_media_buy.mdx
  • docs/media-buy/task-reference/sync_creatives.mdx

Breaking changes

• If you use adcp/client, this change is not breaking (client already maps task data to result).
• If you consume webhook JSON directly, this is a breaking change: task data is now under result.
• error is now a string (only when status = failed).
• domain, created_at, updated_at, completed_at, progress removed from webhook payload.

Migration guide

• Webhook consumers:
  • Read task data from response.result (e.g., response.result.media_buy_id).
  • Read error text from response.error (string) and/or response.message.
• Webhook producers:
  • Do not merge task fields at the root; put all task data under result.
  • Keep protocol fields only at the top level as listed below.

New payload shape (canonical)

{
  "operation_id": "op_123",
  "context_id": "ctx_abc",
  "task_id": "task_456",
  "task_type": "create_media_buy",
  "status": "completed",
  "message": "Media buy created successfully",
  "timestamp": "2025-01-22T10:30:00Z",
  "result": {
    "media_buy_id": "mb_12345",
    "buyer_ref": "nike_q1_campaign_2024",
    "packages": [{ "package_id": "pkg_001" }]
  },
  "error": "optional string when status=failed"
}

Before vs After (compact)

• Before: task fields at root (e.g., media_buy_id, packages)
• After: task fields inside result (e.g., result.media_buy_id, result.packages)