docs: clarify webhook behavior for async operations

[bokelley]

Summary

Documents webhook notification format and trigger behavior for asynchronous AdCP operations, addressing confusion about when webhooks are called and what payload format is sent.

Key Changes

🎯 Webhook Trigger Behavior

• Key rule: Webhooks are ONLY called when initial response is submitted
• completed/failed responses are synchronous - no webhook needed
• working responses complete within ~120s - no webhook, just wait for response
• submitted responses are long-running (hours/days) - webhooks will be used

📦 Webhook Payload Format

• Webhooks POST the complete task response object (not just status notification)
• Each webhook matches the task's response schema exactly
• For create_media_buy: full create-media-buy-response.json structure
• For activate_signal: full activate-signal-response.json structure

🔗 Webhook URL Pattern

• Use task name in URL: /webhooks/adcp/{task_name}/{agent_id}/{operation_id}
• Examples: /create_media_buy/..., /activate_signal/...
• Enables clear routing based on task type

📢 Status Change Notifications

For submitted operations, webhooks are called on:

• input-required → Human needs to respond/approve
• completed → Operation finished successfully
• failed → Operation failed with error details
• canceled → Operation was canceled

Files Modified

• docs/protocols/core-concepts.md - Added comprehensive webhook section with examples
• docs/protocols/task-management.md - Updated webhook integration documentation
• docs/media-buy/task-reference/create_media_buy.md - Clarified webhook flow
• docs/signals/tasks/activate_signal.md - Added webhook examples

Examples Added

• Human-in-the-loop approval flow with webhooks
• Synchronous vs async operation handling
• Complete webhook payload structures matching schemas
• Webhook handler implementation patterns

Test Results

✅ All schema validation tests passed
✅ All example validation tests passed
✅ TypeScript compilation successful
✅ Documentation build successful

[bokelley]

Closing this PR - changes have been consolidated into PR #86 which contains comprehensive webhook documentation including security, reliability, and the async operation behavior clarifications from this PR.