fix(tui): isImageLine should detect image escape sequences anywhere in line

Changed isImageLine() from using startsWith() to includes() to detect
Kitty and iTerm2 image escape sequences anywhere in a line, not just
at the start. This prevents TUI width checks from failing on lines
containing image data, which could cause crashes when rendering tool
results with images (e.g., when reading image files).

Also added comprehensive test coverage for isImageLine() including:
- Both iTerm2 and Kitty protocols
- Regression tests for long lines and terminals without image support
- Negative cases to ensure no false positives

Fixes crash: 'Rendered line exceeds terminal width' when image
escape sequences appear in output.

packages/tui/src/terminal-image.ts

@@ -79,24 +79,12 @@ export function getCapabilities(): TerminalCapabilities {
 
 export function resetCapabilitiesCache(): void {
 	cachedCapabilities = null;
-	imageEscapePrefix = undefined;
-}
-
-let imageEscapePrefix: string | null | undefined;
-
-function getImageEscapePrefix(): string | null {
-	if (imageEscapePrefix === undefined) {
-		const protocol = getCapabilities().images;
-		if (protocol === "kitty") imageEscapePrefix = "\x1b_G";
-		else if (protocol === "iterm2") imageEscapePrefix = "\x1b]1337;File=";
-		else imageEscapePrefix = null;
-	}
-	return imageEscapePrefix;
 }
 
 export function isImageLine(line: string): boolean {
-	const prefix = getImageEscapePrefix();
-	return prefix !== null && line.startsWith(prefix);
+	// Check for Kitty or iTerm2 image escape sequences anywhere in the line
+	// This prevents width checks from failing on lines containing image data
+	return line.includes("\x1b_G") || line.includes("\x1b]1337;File=");
 }
 
 /**