feat: update adcp types for latest push notification schemas and update utility functions around webhooks

[youbek]

Description

This PR updates python generated types after push notification schema changes made to the adcp protocol:

• docs: a2a webhook payload structure and url routing best practices adcp#280
• feat: refactor push notification config spec to align with a2a adcp#268

Also, updates/adds utility functions for handling and creating webhook payloads specifically utilities:

• create_a2a_webhook_payload
• create_mcp_webhook_payload
• get_adcp_signed_headers_for_webhook

Demo (MCP)

• sync creative approval and rejection
  https://www.loom.com/share/930fb382e5404b6b8cd77196f5a398a0

• media buy approval
  https://www.loom.com/share/22ac869407ad4834ac5bae968b3c4237

Demo (A2A)

• media buy approval
  https://www.loom.com/share/7f86b1032164429882d906a3e07540da

• creative approval
  https://www.loom.com/share/11972d1ab2af4672a50363b74c531434

[nastassiafulconis]

Thanks for this PR! The webhook utilities are a great addition. I found a few issues that need addressing before we can merge - mostly around signature verification security. See inline comments below.

[nastassiafulconis]

Thanks for this PR! The webhook utilities are a great addition. Found a few things that need addressing - mostly around signature verification. See inline comments.

[nastassiafulconis]

A few more things I noticed:

client.py signature verification - double check that the verification strips the sha256= prefix before comparing. the header includes it but hmac.compare_digest expects raw hex

tests - solid coverage! one thing that might be worth adding is an end-to-end test that creates a payload with create_mcp_webhook_payload, signs it with get_adcp_signed_headers_for_webhook, then verifies with handle_webhook. would catch any mismatches between the generation and verification sides

breaking changes - might be worth noting in the PR description that the handle_webhook signature changed, in case anyone is already using it