[WIP] Add periodic refresh for notifications every 10 minutes

[Copilot]

Plan: Add Periodic Notification Refresh

• Add new setting key constant for notification refresh interval
• Add configuration in package.json with default of 10 minutes (600 seconds)
• Update NotificationsManager to use the configurable refresh interval
• Remove unused _pollingDuration field
• Fix reference to use _getRefreshInterval()
• Fix disposable leak in _startPolling()
• Handle refresh interval change when polling is not active
• Improve polling lifecycle to prevent resource leaks
• Improve code safety and add better comments
• Run final code review
• Run CodeQL security check

Original prompt

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This section details on the original issue you should resolve

<issue_title>Periodically update notifications</issue_title>
<issue_description>Testing #6329

It seems like the notifications are only update on manual refresh. It would be nice if the extensions would periodically update the notifications
</issue_description>

<agent_instructions>Add a periodic refresh. Set it to 10 minutes by default, controlled by a setting.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Periodically update notifications #6351

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