Improve injection logic and add defensive array checks

[Tarquinen]

Summary

• Add defensive Array.isArray() checks before iterating over msg.parts to prevent runtime errors
• Update injection logic to create a synthetic user message when the last message is from the user (instead of skipping injection entirely), closes Error: Invalid IDs provided. Only use numeric IDs from the <prunable-tools> list. #305
• Update prompt wording to be consistent with new behavior ("turn" instead of "assistant turn", "synthetic message" instead of "assistant message")
• Make discard tool spec guidance tool-agnostic (removes reference to extract tool)