shellIntegrationAddon: fix confusion of 'no match' and 'match at start of string'

deserializeMessage() would not recognize escape sequences at the start
of a string, due to a simple falsey check on `match?.index` which could
be validly 0 instead of null.

In other words: "Foo\x3bBar" would decode to "Foo;Bar", but "\x3b" would
not be recognized as an escape sequence.

This fixes that bug, enabling those suppressed test cases, and disables
the newly-failing tests which had only passed due to the (still-broken)
escaping logic being skipped in those situations.

Author: rwe

src/vs/platform/terminal/common/xterm/shellIntegrationAddon.ts

@@ -517,7 +517,7 @@ export function deserializeMessage(message: string): string {
 	const deserializeRegex = /\\x([0-9a-f]{2})/i;
 	while (true) {
 		const match = result.match(deserializeRegex);
-		if (!match?.index || match.length < 2) {
+		if (match?.index == null || match.length < 2) {
 			break;
 		}
 		result = result.slice(0, match.index) + String.fromCharCode(parseInt(match[1], 16)) + result.slice(match.index + 4);

src/vs/workbench/contrib/terminal/test/browser/xterm/shellIntegrationAddon.test.ts

@@ -288,15 +288,9 @@ suite('deserializeMessage', () => {
 	];
 
 	const BROKEN: readonly string[] = [
-		'escaped semicolon',
-		'escaped semicolon (upper hex)',
-		// only "passing" due to interaction between two separate bugs:
-		// 'escaped backslash followed by literal "x3b" is not a semicolon',
+		'escaped backslash followed by literal "x3b" is not a semicolon',
 		'non-initial escaped backslash followed by literal "x3b" is not a semicolon',
-		'escaped newline',
-		'escaped newline (upper hex)',
-		// only "passing" due to interaction between two separate bugs:
-		// 'escaped backslash followed by literal "x0a" is not a newline',
+		'escaped backslash followed by literal "x0a" is not a newline',
 		'non-initial escaped backslash followed by literal "x0a" is not a newline',
 	];