Add {{agentName}} support to agent response templates

[warengonzaga]

Summary

Add support for a new {{agentName}} template variable so admins can include the responding agent's display name in Telegram agent response messages.

Problem

The current template builder for agent_response does not expose sender information. Admins can customize the message layout, but they cannot show who sent the response from the Unthread dashboard.

Current Behavior

• Agent response templates support variables like {{ticketNumber}} and {{response}}.
• Sender information is not exposed as a template variable.
• The webhook/template rendering path would need to pass a stable display name from the webhook payload into the template renderer.

Desired Behavior

• Support a new variable: {{agentName}}
• Expose it in the template builder documentation/variable list.
• Populate it from the webhook payload when available.
• Fall back safely when the sender name is unavailable or empty.

Suggested Scope

• Add agentName to the available template variables for agent responses.
• Update the webhook message formatting path to pass agentName into template rendering.
• Decide fallback behavior when sender info is missing.
• Optionally update the default agent_response template to use {{agentName}} once the data path is stable.

Acceptance Criteria

• Admins can include {{agentName}} in the agent_response template.
• Rendered Telegram messages include the sender name when available.
• Missing sender names do not break template rendering.
• Tests cover both present and missing sender-name cases.