Add SshSessionBackend test coverage (#1)

Adds two kinds of coverage, continuing #1's remaining scope:

1. Pure-function tests for mapSshError() and escapeForShellSingleQuotes(),
   promoted from private members to public statics purely so tests can
   call them without spinning up a process. escapeForShellSingleQuotes()
   is the actual security boundary for password auth (it's what stops a
   password containing a single quote from breaking out of the askpass
   script's quoting), so it gets a real adversarial test, not just a
   happy-path one.

2. State-machine tests (connect -> Connected, auth failure -> Failed with
   the right mapped message, connection refused -> Failed, input
   round-tripping, reconnect) driven against tests/fixtures/fake_ssh.sh,
   a small controllable stand-in for the real ssh binary, instead of a
   real network/SSH server. This needed one small testability seam: a new
   constructor overload that overrides the launched program ("ssh" in
   production, the fixture script in tests).

POSIX-only for now: the fixture is a shell script, so the state-machine
tests QSKIP on Windows until an equivalent fixture exists there; the
pure-function tests run everywhere.

RdpSessionBackend coverage is still open -- it's a bigger lift again
(FreeRDP's own event loop, not just a QProcess), left for a follow-up.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/ssh_session_backend.cpp

@@ -7,16 +7,14 @@
 #include <QTextStream>
 #include <QUuid>
 
-namespace {
-QString escapeForShellSingleQuotes(const QString& value)
+SshSessionBackend::SshSessionBackend(const Profile& profile, QObject* parent)
+    : SshSessionBackend(profile, QStringLiteral("ssh"), parent)
 {
-    QString escaped = value;
-    escaped.replace(QStringLiteral("'"), QStringLiteral("'\"'\"'"));
-    return escaped;
-}
 }
 
-SshSessionBackend::SshSessionBackend(const Profile& profile, QObject* parent)
+SshSessionBackend::SshSessionBackend(const Profile& profile,
+                                     const QString& sshProgramOverride,
+                                     QObject* parent)
     : SessionBackend(profile, parent),
       m_process(new QProcess(this)),
       m_connectedProbeTimer(new QTimer(this)),
@@ -27,7 +25,8 @@ SshSessionBackend::SshSessionBackend(const Profile& profile, QObject* parent)
       m_waitingForHostKeyConfirmation(false),
       m_passwordSubmitted(false),
       m_terminalColumns(0),
-      m_terminalRows(0)
+      m_terminalRows(0),
+      m_sshProgram(sshProgramOverride)
 {
     m_connectedProbeTimer->setSingleShot(true);
 
@@ -395,7 +394,7 @@ bool SshSessionBackend::startSshProcess(const SessionConnectOptions& options)
     args << target;
 
     m_process->setProcessEnvironment(environment);
-    m_process->setProgram(QStringLiteral("ssh"));
+    m_process->setProgram(m_sshProgram);
     m_process->setArguments(args);
     m_process->setProcessChannelMode(QProcess::SeparateChannels);
 
@@ -471,7 +470,7 @@ void SshSessionBackend::cleanupAskPassScript()
     }
 }
 
-QString SshSessionBackend::mapSshError(const QString& rawError) const
+QString SshSessionBackend::mapSshError(const QString& rawError)
 {
     const QString raw = rawError.trimmed();
     if (raw.contains(QStringLiteral("Permission denied"), Qt::CaseInsensitive)) {
@@ -510,6 +509,13 @@ QString SshSessionBackend::mapSshError(const QString& rawError) const
     return QStringLiteral("SSH connection failed.");
 }
 
+QString SshSessionBackend::escapeForShellSingleQuotes(const QString& value)
+{
+    QString escaped = value;
+    escaped.replace(QStringLiteral("'"), QStringLiteral("'\"'\"'"));
+    return escaped;
+}
+
 QString SshSessionBackend::knownHostsFileForNullDevice() const
 {
 #ifdef Q_OS_WIN