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.h

@@ -13,8 +13,16 @@ class SshSessionBackend : public SessionBackend
 
 public:
     explicit SshSessionBackend(const Profile& profile, QObject* parent = nullptr);
+    // Test-only: overrides the executable launched instead of "ssh", so
+    // tests can point it at a controllable fixture script.
+    SshSessionBackend(const Profile& profile, const QString& sshProgramOverride, QObject* parent);
     ~SshSessionBackend() override;
 
+    // Pure, state-free helpers exposed as public statics purely so tests
+    // can exercise them directly without spinning up a real ssh process.
+    static QString mapSshError(const QString& rawError);
+    static QString escapeForShellSingleQuotes(const QString& value);
+
 public slots:
     void connectSession(const SessionConnectOptions& options) override;
     void disconnectSession() override;
@@ -46,6 +54,7 @@ private:
     bool m_passwordSubmitted;
     int m_terminalColumns;
     int m_terminalRows;
+    QString m_sshProgram;
 
     void setState(SessionState state, const QString& message);
     bool startSshProcess(const SessionConnectOptions& options);
@@ -53,7 +62,6 @@ private:
                           QProcessEnvironment& environment,
                           QString& error);
     void cleanupAskPassScript();
-    QString mapSshError(const QString& rawError) const;
     QString knownHostsFileForNullDevice() const;
     void applyTerminalSizeIfAvailable();
 };