Refactor: Extract session saving logic into private method

[Copilot]

Session saving logic was duplicated between _doOpenTab and removeFromOpenedFiles, creating maintenance burden and potential for inconsistencies.

Changes

• Extracted _saveSession() private method: Encapsulates session persistence logic (preference check, file collection across editor windows, data center update)
• Refactored call sites: Both _doOpenTab and removeFromOpenedFiles now delegate to _saveSession()

Before:

// Duplicated in both methods
const { restoreSession } = preferences.getAll()
if (restoreSession) {
  const allOpenedFiles = []
  const windowManager = _accessor.windowManager
  for (const window of windowManager.windows.values()) {
    if (window.type === 'editor') {
      allOpenedFiles.push(...window.openedFiles)
    }
  }
  dataCenter.setItem('lastSession', allOpenedFiles).then(...)
}

After:

this._saveSession()

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