fix(update,test): clarify wmic comment; switch tests to monkeypatch sys.platform

Two fix-ups for #17123:

1. Reword the inline comment in `_warn_stale_dashboard_processes` to
   accurately describe the failure mode (locale-dependent decoder, not a
   "default UTF-8 decoder") and identify `errors="ignore"` as the
   load-bearing protection. Per Copilot's review.

2. Switch `TestWindowsWmicEncoding` from `patch("hermes_cli.main.sys")`
   to `monkeypatch.setattr(sys, "platform", "win32")` — the codebase's
   canonical pattern (e.g. `tests/hermes_cli/test_auth_ssl_macos.py`).
   The MagicMock-replacement approach passed locally on Python 3.12 but
   the platform-equality check failed under CI's xdist+Python 3.11,
   leaving both new tests red despite the fix being present.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hermes_cli/main.py

@@ -5274,12 +5274,13 @@ def _warn_stale_dashboard_processes() -> None:
 
     try:
         if sys.platform == "win32":
-            # wmic emits text in the system code page (e.g. cp936 on zh-CN
-            # locales), not UTF-8. Without an explicit encoding, Python's
-            # default UTF-8 decoder crashes the subprocess reader thread
-            # with UnicodeDecodeError, leaving result.stdout=None and the
-            # subsequent .split() call crashing `hermes update` with an
-            # AttributeError on Windows non-UTF-8 locales (#17049).
+            # wmic may emit text in the system code page (for example cp936
+            # on zh-CN systems), not UTF-8. In text mode, subprocess output
+            # decoding depends on Python's configuration (locale-dependent
+            # by default, or UTF-8 in UTF-8 mode). The important protection
+            # here is errors="ignore": it prevents a reader-thread
+            # UnicodeDecodeError from leaving result.stdout=None and turning
+            # the later .split() into an AttributeError (#17049).
             result = subprocess.run(
                 ["wmic", "process", "get", "ProcessId,CommandLine",
                  "/FORMAT:LIST"],