Convert Copilot SVGs to data URIs with resolved colors

[Copilot]

Problem

The new Copilot SVGs use CSS color variables (var(--vscode-list-errorForeground, currentColor), etc.) but since they are fetched as external resources, these variables are not applied, resulting in incorrect colors being displayed.

Solution

This PR converts the 3 Copilot status SVGs to data URIs with the CSS color variables resolved to actual color values, ensuring correct colors are used regardless of how the SVGs are loaded.

Changes Made

1. Added data URI conversion functions in src/common/uri.ts:

   • copilotErrorAsImageDataURI() - converts error SVG with resolved red color (#f14c4c)
   • copilotInProgressAsImageDataURI() - converts in-progress SVG with resolved orange color (#ff8c00)
   • copilotSuccessAsImageDataURI() - converts success SVG with resolved green color (#73c991)

2. Updated src/common/resources.ts:

   • Changed copilot icon type definitions from string to vscode.Uri
   • Updated icon initialization to use data URI functions instead of file paths

Color Variable Resolution

CSS Variable	Resolved Color	Usage
var(--vscode-list-errorForeground, currentColor)	#f14c4c	Error icon X symbol
var(--vscode-editorWarning-foreground, currentColor)	#ff8c00	In-progress refresh arrows
var(--vscode-notebookStatusSuccessIcon-foreground, currentColor)	#73c991	Success checkmark

Benefits

• ✅ Icons display with correct colors in all contexts
• ✅ No dependency on external CSS variables
• ✅ Maintains backward compatibility (existing usage sites accept both string and vscode.Uri)
• ✅ Uses existing DataUri.asImageDataURI pattern for consistency

The main copilot.svg was left unchanged as it uses fill="currentColor" which correctly inherits color from context.

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