macOS: support decoding the surrogate pair with UnicodeHexInput (#13737)

Not yet a perfect fix for
https://github.com/ghostty-org/ghostty/discussions/13730

macos/Sources/Ghostty/Surface View/SurfaceView_AppKit.swift

@@ -198,6 +198,8 @@ extension Ghostty {
 
         // This is set to non-null during keyDown to accumulate insertText contents
         private var keyTextAccumulator: [String]?
+        /// Temporary lead surrogate that's waiting for the trail
+        private var leadSurrogate: LeadSurrogate?
 
         // True when we've consumed a left mouse-down only to move focus and
         // should suppress the matching mouse-up from being reported.
@@ -2069,8 +2071,21 @@ extension Ghostty.SurfaceView: NSTextInputClient {
         switch string {
         case let v as NSAttributedString:
             chars = v.string
-        case let v as String:
-            chars = v
+        case let v as NSString:
+            if let leadSurrogate = LeadSurrogate(v) {
+                self.leadSurrogate = leadSurrogate
+                chars = ""
+            } else if let trail = TrailSurrogate(v) {
+                // We ignore trail surrogate without a lead like Terminal.app.
+                chars = leadSurrogate?.encode(trail: trail) ?? ""
+                leadSurrogate = nil
+            } else {
+                chars = v as String
+                // Clear whenever other text got inserted.
+                // Ideally we should encode any adjacent lead and trail surrogate into one,
+                // but getting the cursor position and reading could be rather expensive to do.
+                leadSurrogate = nil
+            }
         default:
             return
         }
@@ -2455,3 +2470,41 @@ class CachedValue<T> {
         expiryTask = nil
     }
 }
+
+/// Check if a UTF16 text is a single lead surrogate character
+struct LeadSurrogate {
+    let char: UTF16Char
+
+    init?(_ text: NSString) {
+        guard text.length == 1 else {
+            return nil
+        }
+        let char = text.character(at: 0)
+        if UTF16.isLeadSurrogate(char) {
+            self.char = char
+        } else {
+            return nil
+        }
+    }
+
+    func encode(trail: TrailSurrogate) -> String {
+        String(decoding: [char, trail.char], as: UTF16.self)
+    }
+}
+
+/// Check if a UTF16 text is a single trail surrogate character
+struct TrailSurrogate {
+    let char: UTF16Char
+
+    init?(_ text: NSString) {
+        guard text.length == 1 else {
+            return nil
+        }
+        let char = text.character(at: 0)
+        if UTF16.isTrailSurrogate(char) {
+            self.char = char
+        } else {
+            return nil
+        }
+    }
+}