Add a "Test webhook" button for webhook automations

[DiegoDeDios]

Describe the feature or potential improvement

When setting up an automation with a webhook action, there's no way to send a test call from the Langfuse UI to confirm the endpoint is reachable and the receiving flow works.

Our use case: we send Langfuse alerts to a Microsoft Power Automate HTTP-trigger flow. Right now, to check that the flow is working, we have to wait for (or manually cause) a real event that triggers the automation. That makes the initial setup slow, and it's also hard to re-check later after the flow or URL changes.

Proposal: add a "Test webhook" / "Send test event" button on the webhook action (when creating/editing the automation and on the existing automation page) that:

• Sends a sample payload with the same shape, headers and signature as a real event for the selected trigger type
• Shows the result in the UI: HTTP status code, response time and the response body (or error)
• Optionally records the test call in the automation's execution history, marked as a test

Many webhook providers (e.g. Stripe, GitHub) offer this, and it would make integrations with tools like Power Automate, n8n, Zapier or Slack much easier to set up and debug.

Additional information

Related: https://github.com/orgs/langfuse/discussions/19058 (custom request body templates for webhook actions). A test button would be even more useful alongside custom bodies, since users could preview what their endpoint actually receives.