Merge pull request #22450 from github/redsun82-skip-mono-integration-tests-on-ubuntu-26-04

Skip mono/nuget C# integration tests on Ubuntu 26.04

Author: redsun82

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

@@ -5,15 +5,10 @@ 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).
-    macOS ARM runners (macos-15 and macos-26) are excluded due to issues with Mono and nuget.
+    Returns True on Ubuntu before 26.04 and on macOS x86_64 before macOS 15. Linux other than
+    Ubuntu is not selected, as we do not test on it.
+    Ubuntu dropped the `mono-complete` package in 26.04, and mono is end-of-life, its own apt
+    repository publishing nothing newer than Ubuntu 20.04, so there is nothing to fall back on.
+    macOS 15 and later are ARM runners, which have issues with Mono and nuget.
     """
-    return (
-        runs_on.linux
-        or (
-            runs_on.macos
-            and runs_on.x86_64
-            and not runs_on.macos_15
-            and not runs_on.macos_26
-        )
-    )
+    return runs_on.ubuntu < 2604 or (runs_on.macos < 15 and runs_on.x86_64)