Extract Windows fallback process creation to separate function

Refactor create_windows_process to improve readability by extracting
the fallback logic into _create_windows_fallback_process. This:

- Reduces nesting and complexity in the main function
- Makes the primary vs fallback paths clearer
- Improves testability of the fallback logic
- Better separates concerns between anyio and subprocess approaches

No functional changes, just code organization.

Author: felixweinberger

src/mcp/client/stdio/win32.py

@@ -159,31 +159,7 @@ async def create_windows_process(
         )
         return process
     except NotImplementedError:
-        # Fall back to subprocess.Popen for SelectorEventLoop compatibility
-        try:
-            popen_obj = subprocess.Popen(
-                [command, *args],
-                stdin=subprocess.PIPE,
-                stdout=subprocess.PIPE,
-                stderr=errlog,
-                env=env,
-                cwd=cwd,
-                bufsize=0,  # Unbuffered output
-                creationflags=getattr(subprocess, "CREATE_NO_WINDOW", 0),
-            )
-            return FallbackProcess(popen_obj)
-        except Exception:
-            # If creationflags failed, try without them
-            popen_obj = subprocess.Popen(
-                [command, *args],
-                stdin=subprocess.PIPE,
-                stdout=subprocess.PIPE,
-                stderr=errlog,
-                env=env,
-                cwd=cwd,
-                bufsize=0,
-            )
-            return FallbackProcess(popen_obj)
+        return _create_windows_fallback_process(command, args, env, errlog, cwd)
     except Exception:
         # If anyio failed for other reasons, try without creation flags
         process = await anyio.open_process(
@@ -193,3 +169,52 @@ async def create_windows_process(
             cwd=cwd,
         )
         return process
+
+
+def _create_windows_fallback_process(
+    command: str,
+    args: list[str],
+    env: dict[str, str] | None = None,
+    errlog: TextIO | None = sys.stderr,
+    cwd: Path | str | None = None,
+) -> FallbackProcess:
+    """
+    Create a FallbackProcess using subprocess.Popen.
+
+    This is used when anyio.open_process is not available (e.g., on SelectorEventLoop).
+
+    Args:
+        command: The executable to run
+        args: List of command line arguments
+        env: Environment variables
+        errlog: Where to send stderr output (defaults to sys.stderr)
+        cwd: Working directory for the subprocess
+
+    Returns:
+        FallbackProcess: Async-compatible subprocess wrapper
+    """
+    try:
+        # Try launching with creationflags to avoid opening a new console window
+        popen_obj = subprocess.Popen(
+            [command, *args],
+            stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE,
+            stderr=errlog,
+            env=env,
+            cwd=cwd,
+            bufsize=0,  # Unbuffered output
+            creationflags=getattr(subprocess, "CREATE_NO_WINDOW", 0),
+        )
+        return FallbackProcess(popen_obj)
+    except Exception:
+        # If creationflags failed, try without them
+        popen_obj = subprocess.Popen(
+            [command, *args],
+            stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE,
+            stderr=errlog,
+            env=env,
+            cwd=cwd,
+            bufsize=0,
+        )
+        return FallbackProcess(popen_obj)