Fix: Make fcntl usage conditional in mcpm share for Windows compatibility

The `fcntl` module is not available on Windows, and its direct import
was causing `ImportError` when `mcpm share` was invoked or its module
(src/mcpm/commands/share.py) was loaded on Windows systems.

This commit makes the import and usage of `fcntl` within the
`make_non_blocking` function conditional on `os.name == 'posix'`.
On non-POSIX systems (like Windows), the function will now do nothing.

The existing code for reading from subprocess pipes in `mcpm share`
utilizes `select.select()` with timeouts and handles `IOError`/`OSError`
exceptions. This setup is expected to provide sufficient non-blocking
behavior for reading subprocess output on Windows, even without an
explicit `fcntl` call to set `O_NONBLOCK`.

Note: Full dynamic testing of `mcpm share` on a Windows-like environment
was not possible due to a Python version mismatch (environment: 3.10.17,
project requires: >=3.11) in the available testing sandbox. This change
addresses the direct `ImportError`.

Author: google-labs-jules[bot]

src/mcpm/commands/share.py

@@ -29,11 +29,15 @@ def find_mcp_proxy() -> Optional[str]:
 
 def make_non_blocking(file_obj):
     """Make a file object non-blocking."""
-    import fcntl
-
-    fd = file_obj.fileno()
-    fl = fcntl.fcntl(fd, fcntl.F_GETFL)
-    fcntl.fcntl(fd, fcntl.F_SETFL, fl | os.O_NONBLOCK)
+    if os.name == 'posix':
+        import fcntl
+
+        fd = file_obj.fileno()
+        fl = fcntl.fcntl(fd, fcntl.F_GETFL)
+        fcntl.fcntl(fd, fcntl.F_SETFL, fl | os.O_NONBLOCK)
+    # On other platforms (e.g., Windows), we rely on the behavior of select()
+    # and the non-blocking nature of readline() on Popen streams,
+    # or the existing try-except for IOError/OSError.
 
 
 def wait_for_random_port(process: subprocess.Popen, timeout: int = 20) -> Optional[int]: