fix(cdp): backslash-escape key/code in Input.dispatchKeyEvent (#433) (#437)

The keyDown/keyUp paths interpolated `key`/`code` into the generated
KeyboardEvent snippet with single-quote escaping only. Chrome sends
`key: "\"` (U+005C) for the backslash key, which became `key:'\'` — the
backslash escaped the closing quote, ran the string literal on, and made
the whole page.evaluate a syntax error, so the keydown was silently
dropped. Any key/code carrying a backslash broke dispatch.

Escape backslash before single-quote (backslash first, so the quote-escape
backslashes are not themselves doubled), matching the sibling text path in
the same function. Adds a regression test asserting the backslash key
arrives at a keydown listener alongside an ordinary key.

crates/obscura-cdp/src/domains/input.rs

@@ -151,8 +151,12 @@ pub async fn handle(
                                 var evt = globalThis.__obscura_markTrusted(new KeyboardEvent('keydown', {{bubbles:true,cancelable:true,key:'{key}',code:'{code}'}}));\
                                 target.dispatchEvent(evt);\
                             }})()",
-                            key = key.replace('\'', "\\'"),
-                            code = code.replace('\'', "\\'"),
+                            // Escape backslash BEFORE single-quote (as the text
+                            // path below does) so a key like "\" — Chrome's
+                            // backslash key — doesn't escape the closing quote
+                            // and produce a syntax error that drops the event.
+                            key = key.replace('\\', "\\\\").replace('\'', "\\'"),
+                            code = code.replace('\\', "\\\\").replace('\'', "\\'"),
                         );
                         page.evaluate(&js);
 
@@ -194,8 +198,8 @@ pub async fn handle(
                                 var evt = globalThis.__obscura_markTrusted(new KeyboardEvent('keyup', {{bubbles:true,key:'{key}',code:'{code}'}}));\
                                 target.dispatchEvent(evt);\
                             }})()",
-                            key = key.replace('\'', "\\'"),
-                            code = code.replace('\'', "\\'"),
+                            key = key.replace('\\', "\\\\").replace('\'', "\\'"),
+                            code = code.replace('\\', "\\\\").replace('\'', "\\'"),
                         );
                         page.evaluate(&js);
                     }