[Feat]: Send unique token in push notification header #284

Is your feature request related to a problem? Please describe.

As suggested by A2A specification, push notification events should send unique tokens.
ref

Describe the solution you'd like

As suggested by the specification:

The server SHOULD include this token in the notification request (e.g., in a custom header like X-A2A-Notification-Token) for validation by the client's webhook receiver.

Describe alternatives you've considered

No response

Additional context

No response

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