GH-134453: Fix subprocess memoryview input handling on POSIX (GH-134949)

Fix inconsistent subprocess.Popen.communicate() behavior between Windows
and POSIX when using memoryview objects with non-byte elements as input.

On POSIX systems, the code was incorrectly comparing bytes written against
element count instead of byte count, causing data truncation for large
inputs with non-byte element types.

Changes:
- Cast memoryview inputs to byte view when input is already a memoryview
- Fix progress tracking to use len(input_view) instead of len(self._input)
- Add comprehensive test coverage for memoryview inputs

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* old-man-yells-at-ReST
* Update 2025-05-30-18-37-44.gh-issue-134453.kxkA-o.rst
* assertIsNone review feedback
* fix memoryview_nonbytes test to fail without our fix on main, and have a nicer error.

Thanks to Peter Bierma @ZeroIntensity for the code review.

Lib/subprocess.py

@@ -2102,7 +2102,10 @@ def _communicate(self, input, endtime, orig_timeout):
             self._save_input(input)
 
             if self._input:
-                input_view = memoryview(self._input)
+                if not isinstance(self._input, memoryview):
+                    input_view = memoryview(self._input)
+                else:
+                    input_view = self._input.cast("b")  # byte input required
 
             with _PopenSelector() as selector:
                 if self.stdin and not self.stdin.closed and self._input:
@@ -2138,7 +2141,7 @@ def _communicate(self, input, endtime, orig_timeout):
                                 selector.unregister(key.fileobj)
                                 key.fileobj.close()
                             else:
-                                if self._input_offset >= len(self._input):
+                                if self._input_offset >= len(input_view):
                                     selector.unregister(key.fileobj)
                                     key.fileobj.close()
                         elif key.fileobj in (self.stdout, self.stderr):