feat(ecosystem): Implement cross-system issue synchronization

[ShashankFC]

Test 7

Summary by CodeRabbit

Release Notes

• New Features
  • Enhanced assignment synchronization with cycle prevention to ensure reliable bidirectional syncing between Sentry and integrated external issue tracking systems without infinite loops.

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Replicated from ai-code-review-evaluation/sentry-coderabbit#7

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Note

Medium Risk
Touches core assignment and integration sync pathways and changes when outbound sync is suppressed, which could lead to missed assignee updates if source detection/propagation is incorrect.

Overview
Adds an AssignmentSource payload that is threaded through group assignment/deassignment and outbound integration sync tasks to track where an assignment change originated.

Updates IssueSyncIntegration.should_sync (and related call sites) to skip outbound assignee sync when the change source matches the target integration, preventing infinite sync loops while still allowing other integrations to propagate changes. Includes new unit tests for AssignmentSource serialization and for the cycle-prevention behavior in GroupAssignee outbound sync.

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[ShashankFC]

bugbot run

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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Dataclass default timezone.now() evaluated once at import time

High Severity

The queued field default timezone.now() is evaluated once when the module is first imported, not each time an AssignmentSource instance is created. Every instance that doesn't explicitly provide queued (including all calls via from_integration) will share the same stale timestamp from server startup. The fix is to use field(default_factory=timezone.now) from dataclasses.