Skip mono/nuget C# integration tests on Ubuntu 26.04

Mono is end-of-life and its apt repository only ships Ubuntu 18.04 packages,
whose gtk2-era dependencies cannot be satisfied on 26.04. Without mono the
legacy `packages.config` restore silently yields nothing and the buildless
extractor falls back to downloading .NET Framework reference assemblies, so
these four tests fail rather than being skipped.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: redsun82

csharp/ql/integration-tests/posix/conftest.py

@@ -5,11 +5,14 @@ def _supports_mono_nuget():
     """
     Helper function to determine if the current platform supports Mono and nuget.
     
-    Returns True if running on Linux or on macOS x86_64 (excluding macos-15 and macos-26).
+    Returns True if running on Linux (excluding Ubuntu 26.04) or on macOS x86_64 (excluding
+    macos-15 and macos-26).
+    Mono is end-of-life and only publishes packages up to Ubuntu 18.04, whose dependencies are
+    unsatisfiable from Ubuntu 26.04 on.
     macOS ARM runners (macos-15 and macos-26) are excluded due to issues with Mono and nuget.
     """
     return (
-        runs_on.linux
+        (runs_on.linux and not runs_on.ubuntu_26_04)
         or (
             runs_on.macos
             and runs_on.x86_64