Undefined-behaviour fix: safely call std::isspace in CompletionData (microsoft#5564)

### What was fixed

[CompletionData.cpp](cci:7://file:///c:/Users/T2430514/Downloads/winget-cli/src/AppInstallerCLICore/CompletionData.cpp:0:0-0:0)
passed a potentially negative `char` to `std::isspace`, invoking
undefined behaviour on non-ASCII input (signed `char` platforms). This
occasionally caused crashes or incorrect cursor repositioning when
autocompleting commands containing UTF-8 characters.

### How it was fixed
The character is now explicitly cast to `unsigned char` before the
`std::isspace` check:

```cpp
!std::isspace(static_cast<unsigned char>(commandLine[cursor - 1]))
```

This aligns with standard C++ guidance and makes the completion logic
robust across all locales.
###### Microsoft Reviewers: [Open in
CodeFlow](https://microsoft.github.io/open-pr/?codeflow=https://github.com/microsoft/winget-cli/pull/5564)

Co-authored-by: Odio Marcelino <[email protected]>

Author: mohiuddin-khan-shiam

src/AppInstallerCLICore/CompletionData.cpp

@@ -59,7 +59,7 @@ namespace AppInstaller::CLI
             // very few users are likely to put any spaces at the front of their statements, let alone many.
             else
             {
-                for (; cursor > 0 && !std::isspace(commandLine[cursor - 1]); --cursor);
+                for (; cursor > 0 && !std::isspace(static_cast<unsigned char>(commandLine[cursor - 1])); --cursor);
 
                 AICLI_LOG(CLI, Info, << "Cursor position moved to '" << cursor << '\'');