Add Voicemail Status Entity for Real-time Voicemail Monitoring

[Copilot]

This PR implements a new voicemail status entity that provides real-time monitoring of voicemail messages for each configured extension, addressing the feature request for voicemail functionality using Asterisk's AMI interface.

Features Added

• VoicemailStatusSensor: A new sensor entity that displays the number of new voicemail messages
• Real-time Updates: Automatically updates when voicemails are received or retrieved via MWI events
• Comprehensive Attributes: Provides detailed information including new messages, old messages, and total count
• Dynamic Icons: Shows voicemail icon when messages exist, email-outline when empty
• Error Handling: Robust error handling for malformed AMI responses

Technical Implementation

The implementation leverages Asterisk's built-in AMI capabilities:

• Uses MailboxStatus AMI action to retrieve initial voicemail counts
• Listens for MWI (Message Waiting Indicator) events for real-time updates
• Follows existing sensor patterns in the integration
• Uses mailbox format extension@default (e.g., "100@default")

Example Usage

Once configured, users will automatically get a voicemail status sensor for each extension:

Entity: sensor.100_voicemail_status
State: 2  # Number of new voicemails
Attributes:
  - new_messages: 2
  - old_messages: 5
  - total_messages: 7
  - mailbox: "100@default"

Testing

• Added comprehensive test coverage for the new sensor
• Enhanced mock AMI client to support MailboxStatus actions
• All existing tests continue to pass
• Code formatted and linted per project standards

Fixes #108.

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