Header text partially highlighted

[ctchen222]

When using custom headers with emojis (e.g., "👤 User "), parts of the header text are incorrectly highlighted with different colors instead of using the consistent
CopilotChatHeader highlight group.

Steps to Reproduce

1. Configure custom headers with emojis:

   require("CopilotChat").setup({
     headers = {
       user = "👤 User ",
       assistant = "🤖 Assistant ",
       tool = "🔧 Tool ",
     },
   })

2. Open CopilotChat and start a conversation
3. Observe the header text

Expected Behavior

The entire header text should use the CopilotChatHeader highlight group consistently.

Actual Behavior

Parts of the header text (e.g., "Us" in "👤 User") are highlighted with different colors (often orange/Function color), while other parts use the correct header color or remain
uncolored.