fix(gateway): tolerate non-UTF-8 status/pid files in gateway status reads

`_read_json_file` caught OSError but not UnicodeDecodeError, so a status
file holding binary/non-UTF-8 bytes (truncated or clobbered write) would
crash the gateway status path instead of being treated as unreadable.
UnicodeDecodeError is a ValueError subclass, not an OSError, so it
escaped the existing guard.

Widen the catch to (OSError, UnicodeDecodeError) at both read sites in
gateway/status.py — `_read_json_file` and the sibling `_read_pid_record`,
which had the identical gap. Adds tests covering binary input (returns
None) and valid input (still parses) for both.

Co-authored-by: Teknium <127238744+teknium1@users.noreply.github.com>

gateway/status.py

@@ -227,7 +227,10 @@ def _read_json_file(path: Path) -> Optional[dict[str, Any]]:
         return None
     try:
         raw = path.read_text(encoding="utf-8").strip()
-    except OSError:
+    except (OSError, UnicodeDecodeError):
+        # OSError: file vanished or permission flipped between exists() and
+        # read. UnicodeDecodeError: file holds non-UTF-8 / binary garbage
+        # (a truncated or clobbered status file). Either way it's unusable.
         return None
     if not raw:
         return None
@@ -249,8 +252,9 @@ def _read_pid_record(pid_path: Optional[Path] = None) -> Optional[dict]:
 
     try:
         raw = pid_path.read_text().strip()
-    except OSError:
-        # File was deleted between exists() and read_text(), or permission flipped.
+    except (OSError, UnicodeDecodeError):
+        # File was deleted between exists() and read_text(), permission
+        # flipped, or it holds non-UTF-8 / binary garbage.
         return None
     if not raw:
         return None