macOS: update default behaviour of KeyboardLayout.character(for:modifiers:) (#14009)

Follow up for #13888, and prepare for #13205.

The comments are copied from the history commit.

## AI Disclosure

The tests are updated by Claude, I cherrypicked them.

macos/Sources/Ghostty/Ghostty.Input.swift

@@ -29,9 +29,11 @@ extension Ghostty {
                     Self.writingSystemKeyRange.contains(physical.rawValue),
                     let inputKey = Input.Key(cKey: physical),
                     let keyCode = inputKey.keyCode,
+                    // Command can select a distinct layout table. Other modifiers remain
+                    // separate in the menu's modifier mask and must not affect this character.
                     let character = KeyboardLayout.character(
                         for: keyCode,
-                        modifiers: modifierFlags)
+                        modifiers: modifierFlags.intersection(.command))
                 else { return nil }
 
                 // Printable physical keys must be translated through the current layout.

macos/Sources/Helpers/KeyboardLayout.swift

@@ -16,6 +16,8 @@ class KeyboardLayout {
     /// Translate a physical keycode for use as a menu key equivalent.
     ///
     /// AppKit retranslates against the current input source without changing its dead key state.
+    ///
+    /// - Important: Must be called on the main thread because underlying Text Input Sources APIs are not thread-safe.
     @MainActor static func character(
         for keyCode: UInt16,
         modifiers: NSEvent.ModifierFlags
@@ -32,7 +34,7 @@ class KeyboardLayout {
                 charactersIgnoringModifiers: "",
                 isARepeat: false,
                 keyCode: keyCode),
-            let result = event.characters(byApplyingModifiers: modifiers.intersection(.command)),
+            let result = event.characters(byApplyingModifiers: modifiers),
             result.count == 1
         else { return nil }
 

macos/Tests/Helpers/KeyboardLayoutTests.swift

@@ -17,21 +17,11 @@ struct KeyboardLayoutTests {
         #expect(KeyboardLayout.character(for: UInt16.max, modifiers: []) == nil)
     }
 
-    @Test(arguments: [
-        ([.shift, .control, .option], []),
-        ([.command, .shift, .control, .option], .command),
-    ] as [(NSEvent.ModifierFlags, NSEvent.ModifierFlags)])
-    func characterUsesOnlyCommandModifier(
-        modifiers: NSEvent.ModifierFlags,
-        effectiveModifiers: NSEvent.ModifierFlags
-    ) throws {
+    @Test func characterAppliesModifiers() throws {
         let keyCode: UInt16 = 0x00 // W3C KeyA
-        let expected = try #require(KeyboardLayout.character(
-            for: keyCode,
-            modifiers: effectiveModifiers))
-        let actual = try #require(KeyboardLayout.character(
-            for: keyCode,
-            modifiers: modifiers))
-        #expect(actual == expected)
+        let unmodified = try #require(KeyboardLayout.character(for: keyCode, modifiers: []))
+        let shifted = try #require(KeyboardLayout.character(for: keyCode, modifiers: .shift))
+        #expect(shifted != unmodified)
+        #expect(String(shifted).lowercased() == String(unmodified).lowercased())
     }
 }