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>

tests/fixtures/fake_ssh.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+# Minimal, deterministic stand-in for the real `ssh` binary, used by
+# SshSessionBackend's state-machine tests so they never touch a real
+# network or SSH server. Behavior is selected by which fixture hostname
+# appears among argv (SshSessionBackend always passes the profile's
+# host, optionally as user@host, as the final argument).
+for arg in "$@"; do
+    case "$arg" in
+        *@succeed|succeed)
+            echo "Welcome to the fake host."
+            # Stay alive echoing stdin back (simulates an interactive
+            # session) until the backend terminates us.
+            while IFS= read -r line; do
+                echo "$line"
+            done
+            exit 0
+            ;;
+        *@fail-auth|fail-auth)
+            echo "Permission denied (publickey,password)." >&2
+            exit 255
+            ;;
+        *@refuse|refuse)
+            echo "ssh: connect to host refuse port 22: Connection refused" >&2
+            exit 255
+            ;;
+    esac
+done
+echo "fake_ssh.sh: no recognized fixture host in arguments: $*" >&2
+exit 1