Report unavailable protocols and services; send combos as real keys

Diagnostics collects what this session doesn't allow, logs it, and lists
it in the settings panel (with a dot on the gear): KWin not launching
sloposk as its keyboard (detected by WAYLAND_SOCKET, since KWin offers the
input-method global to every client), fake input missing or too old,
org.kde.keyboard unreachable, and the D-Bus control name taken.

Modifier combos go through fake input even in text fields: KWin drops
the input method's modifier mask for most clients, so Ctrl+A typed "a".

The desktop file is generated with an absolute Exec, which KWin needs to
match it: one for the installed binary, one for the build's.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/CMakeLists.txt

@@ -16,6 +16,7 @@ sloposk_add_test(tst_textrules)
 sloposk_add_test(tst_deadkeys)
 sloposk_add_test(tst_keyrepeater)
 sloposk_add_test(tst_visibility)
+sloposk_add_test(tst_diagnostics)
 
 # Needs the QML module itself, not just the C++ backing library.
 sloposk_add_test(tst_keyboardsurface)

tests/tst_diagnostics.cpp

@@ -0,0 +1,46 @@
+#include "diagnostics/Diagnostics.h"
+
+#include <QSignalSpy>
+#include <QTest>
+
+class TestDiagnostics : public QObject
+{
+    Q_OBJECT
+
+private Q_SLOTS:
+    void reportsAndClears()
+    {
+        Diagnostics diagnostics;
+        QSignalSpy changed(&diagnostics, &Diagnostics::problemsChanged);
+        QTest::ignoreMessage(QtWarningMsg, "broken");
+        diagnostics.check(QStringLiteral("a"), false, QStringLiteral("broken"));
+        QCOMPARE(diagnostics.problems(), QStringList{QStringLiteral("broken")});
+        diagnostics.check(QStringLiteral("a"), true, QStringLiteral("broken"));
+        QVERIFY(diagnostics.problems().isEmpty());
+        QCOMPARE(changed.count(), 2);
+    }
+
+    void repeatsAreQuiet()
+    {
+        Diagnostics diagnostics;
+        QSignalSpy changed(&diagnostics, &Diagnostics::problemsChanged);
+        diagnostics.check(QStringLiteral("a"), true, QStringLiteral("fine"));
+        QTest::ignoreMessage(QtWarningMsg, "broken");
+        diagnostics.check(QStringLiteral("a"), false, QStringLiteral("broken"));
+        diagnostics.check(QStringLiteral("a"), false, QStringLiteral("broken")); // no second warning
+        QCOMPARE(changed.count(), 1);
+    }
+
+    void listsByIdNotArrival()
+    {
+        Diagnostics diagnostics;
+        QTest::ignoreMessage(QtWarningMsg, "second");
+        QTest::ignoreMessage(QtWarningMsg, "first");
+        diagnostics.check(QStringLiteral("b"), false, QStringLiteral("second"));
+        diagnostics.check(QStringLiteral("a"), false, QStringLiteral("first"));
+        QCOMPARE(diagnostics.problems(), (QStringList{QStringLiteral("first"), QStringLiteral("second")}));
+    }
+};
+
+QTEST_GUILESS_MAIN(TestDiagnostics)
+#include "tst_diagnostics.moc"