End a string sequence at an ESC that does not start ST

An ESC inside a DCS, OSC or APC ends the string and introduces a sequence of
its own. Giving up at that point left the payload to be typed into the query,
and scanning on to a later ST would swallow that following sequence.

https://vt100.net/emu/dec_ansi_parser

Reported by Copilot on #4926.

src/tui/light.go

@@ -379,12 +379,16 @@ func stringEnd(buffer []byte) int {
 				return i + 1
 			}
 		case Esc.Byte():
-			if i+1 < len(buffer) && buffer[i+1] == '\\' {
+			if i+1 == len(buffer) {
+				return 0 // ST may still be arriving
+			}
+			if buffer[i+1] == '\\' {
 				return i + 2
 			}
-			// A lone ESC is either a truncated payload or a terminator still
-			// arriving. Either way the sequence is not framed.
-			return 0
+			// Any other ESC ends the string and introduces a sequence of its
+			// own, so frame only what precedes it and leave the ESC to be
+			// parsed again. Scanning past it would swallow that sequence too.
+			return i
 		}
 	}
 	return 0

src/tui/light_csi_test.go

@@ -29,7 +29,8 @@ func TestUnknownCSISequence(t *testing.T) {
 		{"\x1b]0;a title\a", Invalid, 12},                 // BEL terminated
 		{"\x1bP>|kitty(0.48.2)\x1b\\", Invalid, 19},       // XTVERSION reply
 		{"\x1b_Gi=1;OK\x1b\\", Invalid, 11},
-		{"\x1bP\ax\x1b\\", Invalid, 6}, // BEL inside a DCS payload is not a terminator               // kitty graphics reply
+		{"\x1bP\ax\x1b\\", Invalid, 6},      // BEL inside a DCS payload is not a terminator
+		{"\x1b]foo\x1bX\x1b\\", Invalid, 5}, // ESC aborts the string, consuming only "\e]foo"               // kitty graphics reply
 
 		// Left alone: this is how ALT-[ and ALT-] arrive
 		{"\x1b[a", Alt, 2},