Update server events from OpenAPI schema

[sahilsuman933]

Description

• Updated server event examples and fields in fern/server-url/events.mdx to align with the OpenAPI schema.
• Added a "Common metadata" section for shared event fields.
• Modified end-of-call-report to use an artifact object for recording, transcript, and messages.
• Included messagesOpenAIFormatted in conversation-update and knowledge-base-request events.
• Added isFiltered, detectedThreats, and originalTranscript fields to the transcript event.
• Added the language field to the language-change-detected event.
• Included a hang-up example for the phone-call-control event.

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work

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[cursor]

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