fix: fix old plugin not exit normally

[fatelei]

Description

close #698

Type of Change

• Bug fix
• New feature
• Refactor
• Performance improvement
• Other

Essential Checklist

Testing

• I have tested the changes locally and confirmed they work as expected
• I have added unit tests where necessary and they pass successfully

Bug Fix (if applicable)

• I have used GitHub syntax to close the related issue (e.g., Fixes #123 or Closes #123)

Additional Information

Please provide any additional context that would help reviewers understand the changes.

[gemini-code-assist]

Code Review

This pull request refactors the local plugin shutdown process to ensure subprocesses are correctly terminated and to prevent process leaks. Key changes include moving the runtime deletion to the OnRuntimeCloseImpl callback, introducing the PluginShutdownManager interface, and implementing a timeout-based shutdown sequence in RemovePluginIfNeeded that falls back to a forceful shutdown if the graceful attempt fails. The review feedback identifies opportunities to improve the robustness of this logic by treating 'not found' errors as successes during shutdown attempts, which prevents unnecessary task failures if a plugin has already been cleaned up or finishes shutting down just as a timeout occurs.