fix: run fish completion on fish 3.x and give Windows a native MCPP path

The fish adapter asked commandline for --tokens-expanded, a flag fish only
gained in 4.0; the completion job's Ubuntu image ships fish 3.7, where the
unknown flag left the word list empty and no candidate was produced. Ask for
--tokenize (-o), which both lines accept and whose cut-at-cursor behaviour is
the same.

The Windows PowerShell job inherits MCPP from a Git Bash environment, so the
value is /d/a/... rather than a native path and native Python resolved
D:\d\a\... . Convert that spelling before resolving the binary.

Author: ZheFeng7110

src/shell.cppm

@@ -66,7 +66,8 @@ compdef _mcpp_complete mcpp mcpp.exe
 )MCPP";
     if (shell == "fish") return R"MCPP(# mcpp completion; source this file from config.fish.
 function __mcpp_candidates
-    set -l words (commandline -xpc)
+    # --tokenize (-o) exists in fish 3.x; --tokens-expanded (-x) is 4.0+.
+    set -l words (commandline -opc)
     set -l current (commandline -ct)
     set -l reply (command $words[1] __complete $words[2..-1] "__mcpp_word__$current" 2>/dev/null)
     if test "$reply[1]" = files

tests/scripts/test_shell_completion.py

@@ -11,8 +11,15 @@
 import unittest
 
 
+def native_path(value):
+    """Make a path exported by Git Bash usable by native Windows Python."""
+    if os.name == "nt" and value[:1] == "/" and value[2:3] == "/":
+        return value[1].upper() + ":" + value[2:]
+    return value
+
+
 ROOT = Path(__file__).resolve().parents[2]
-MCPP = Path(os.environ["MCPP"]).resolve() if os.environ.get("MCPP") else None
+MCPP = Path(native_path(os.environ["MCPP"])).resolve() if os.environ.get("MCPP") else None
 
 
 @unittest.skipUnless(MCPP, "set MCPP to a freshly built binary")