Handle marketplace update failures gracefully in batch operations

[Copilot]

The updateAllMarketplaces function terminated on first failure, preventing remaining marketplaces from updating and never printing the summary message.

Changes:

• Refactored error handling: runGitCommand now throws instead of calling process.exit(1)
• Batch operation resilience: updateAllMarketplaces catches errors per-marketplace, continues processing, and reports summary:

  Error updating beta: Failed to clone/update marketplace: git pull failed
  Updated marketplace: gamma
  
  Update complete: 2 succeeded, 1 failed
  

• Consistent error propagation: Single operations (addMarketplace) handle errors and exit; batch-called operations (updateMarketplace) throw for caller handling
• Test coverage: Added test verifying all marketplaces are attempted despite failures

Bonus fix: Removed duplicate await saveConfig({ line in plugin.ts from previous commit.

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