Refactor check_stale_issues for better readability

[myaple]

Refactored check_stale_issues in src/polling.rs to improve code health.

• Extracted determine_last_activity to handle timestamp calculation and note fetching logic.
• Extracted manage_stale_label to handle the side effects of updating issue labels.
• Simplified the main check_stale_issues loop.
• Verified changes with existing tests in src/tests/polling_tests.rs.

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PR created automatically by Jules for task 750441959531310367 started by @myaple

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