Azure: fall back to powershell when no preferred shell is set (#8197)

I am still not sure what is the full set of scenarios that the problem
might occur, but for me it occurred for an "old" cloud shell account,
and didn't reproduce since I have reconfigured it. These behavior might
be explained by the fact that "preferred shell type" did not exist in
the API originally and thus was not set. In such case, Terminal
succeeds to retrieve to the settings but then crashes when reading the
missing field.  To fix it, I handle the case where the field is missing
and fallback to PowerShell.

## Validation Steps Performed
* Tested manually, only once.

Closes #7056

src/cascadia/TerminalConnection/AzureConnection.cpp

@@ -687,6 +687,25 @@ namespace winrt::Microsoft::Terminal::TerminalConnection::implementation
         _state = AzureState::TermConnecting;
     }
 
+    // Method description:
+    // - Helper function to parse the preferred shell type from user settings returned by cloud console API.
+    // We need this function because the field might be missing in the settings
+    // created with old versions of cloud console API.
+    std::optional<utility::string_t> AzureConnection::_ParsePreferredShellType(const web::json::value& settingsResponse)
+    {
+        if (settingsResponse.has_object_field(L"properties"))
+        {
+            const auto userSettings = settingsResponse.at(L"properties");
+            if (userSettings.has_string_field(L"preferredShellType"))
+            {
+                const auto preferredShellTypeValue = userSettings.at(L"preferredShellType");
+                return preferredShellTypeValue.as_string();
+            }
+        }
+
+        return std::nullopt;
+    }
+
     // Method description:
     // - helper function to connect the user to the Azure cloud shell
     void AzureConnection::_RunConnectState()
@@ -706,9 +725,9 @@ namespace winrt::Microsoft::Terminal::TerminalConnection::implementation
         _WriteStringWithNewline(RS_(L"AzureSuccess"));
 
         // Request for a terminal for said cloud shell
-        const auto shellType = settingsResponse.at(L"properties").at(L"preferredShellType").as_string();
+        const auto shellType = _ParsePreferredShellType(settingsResponse);
         _WriteStringWithNewline(RS_(L"AzureRequestingTerminal"));
-        const auto socketUri = _GetTerminal(shellType);
+        const auto socketUri = _GetTerminal(shellType.value_or(L"pwsh"));
         _TerminalOutputHandlers(L"\r\n");
 
         // Step 8: connecting to said terminal

src/cascadia/TerminalConnection/AzureConnection.h

@@ -95,6 +95,8 @@ namespace winrt::Microsoft::Terminal::TerminalConnection::implementation
         std::optional<std::wstring> _ReadUserInput(InputMode mode);
 
         web::websockets::client::websocket_client _cloudShellSocket;
+
+        static std::optional<utility::string_t> _ParsePreferredShellType(const web::json::value& settingsResponse);
     };
 }