feat: update webhook delivery function to support both mcp and a2a payloads

[youbek]

Main changes

• updates push notification delivery. It now uses official adcp python library for creating payloads depending on the protocol.
• It fixes creative approval/rejection to be fully aligned with adcp protocol.
• It fixes sending correct status when manual approval is required. Previously it was sending completed statuses even the media buy required approval. It led bugs where buyer recieved completed result whereas it was still pending approval on the sales agent part

Related PRs

• adcp-python-client - feat: update adcp types for latest push notification schemas and update utility functions around webhooks adcp-client-python#97
• adcp - feat: refactor push notification config spec to align with a2a adcp#268

Demo Looms (MCP)

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

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

Demo Looms (A2A)

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

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

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[BaiyuScope3]

LGTM

[github-actions]

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