refactor: simplify nested map access and improve readability

[lkk214]

Description

• Eliminate fragile type conversions and simplify access to nested properties
• Fixed the issue that inline cannot get the model list
  

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[sestinj]

@lkk214 I really like this update! Unfortunately before I saw it, we merged another PR that made related changes. I do like the way that you did it more. Would you be willing to resolve the merge conflicts, and then I'll merge?

[lkk214]

@sestinj Before submitting this PR, I was confused and therefore didn't change any other relevant code except the drop-down menu. Actually inline doesn't work as expected, I'm raising the issue now.

1. Does the drop-down menu option value in inline come from config.modelsByRole.edit, or from config.modelsByRole.chat, or config.modelsByRole.edit ?? config.modelsByRole.chat
   (Normally it should be config.modelsByRole.edit, and use config.modelsByRole.chat if config.modelsByRole.edit does not exist, but streamDiffLines only uses config.selectedModelByRole.chat,
   

   continue/core/core.ts

   Lines 77 to 82 in 8057b2a

   	const llm = config.selectedModelByRole.chat;
   	if (!llm) {
   	throw new Error("No chat model selected");
   	}

   
   and is the edit role necessarily also the chat role?)

2. Selecting a model in the drop-down menu does not work, it does not change the value of selectedModelByRole because it does not send the config/updateSelectedModel message to the core.
   So does the IDE need to send a message when a model is selected, or use this code in streamDiffLines?
   const llm = msg.data.modelTitle ?? config.selectedModelByRole.chat