fix: prevent webhook_endpoint extra field from being sent as HTTP header in DiscordWebhookHook

[charmikadi]

When using DiscordWebhookHook with a connection that stores webhook_endpoint in the extra field, Airflow 3.2 now injects remaining connection extra fields as HTTP request headers via HttpHook.get_conn(). This causes Discord to
reject the request with 400 Bad Request because webhook_endpoint is not a valid HTTP header.

The fix overrides get_conn in DiscordWebhookHook to remove webhook_endpoint from the connection's extra fields before HttpHook processes them. Since webhook_endpoint is used to build the request URL, not as an HTTP header,
it should never be sent to Discord. The original extra is restored in a finally block to avoid any side effects.

Closes #65279

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