[WIP] Add periodic refresh for notifications every 10 minutes

[Copilot]

Plan: Add Periodic Notification Refresh

• [x] Add new setting key constant for notification refresh interval
• [x] Add configuration in package.json with default of 10 minutes (600 seconds)
• [x] Update NotificationsManager to use the configurable refresh interval
• [x] Remove unused _pollingDuration field
• [x] Fix reference to use _getRefreshInterval()
• [x] Fix disposable leak in _startPolling()
• [x] Handle refresh interval change when polling is not active
• [x] Improve polling lifecycle to prevent resource leaks
• [x] 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 microsoft/vscode-pull-request-github#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 microsoft/vscode-pull-request-github#6351

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