fix(macos,process_info): tighten error handling and add endian assert

- Guard against `getProcess` returning null in the macOS FFI path
  (`proc_pidpath` can fail with EPERM or for an already-exited PID).
  Stop unwrapping with `!` and only populate the ProcessInfoCache on
  success; the previous code threw NullThrownError there, which the
  outer try/catch silently swallowed and prevented caching of valid
  later results.
- Add a 5-second negative cache for ports whose owner can't be
  resolved (typical for short-lived clients that exited before we
  scanned). Without it every such request triggered a full PID list
  rescan, defeating the point of the pid cache for the failure path.
- Add a debug-only assert that the host is little-endian. The libproc
  parsing reads `(int)htons(port)` by treating the low two bytes as a
  big-endian uint16, which requires a little-endian host. All shipping
  macOS hardware satisfies this; the assert fails loudly if the
  assumption is ever broken (stripped from release builds).

Re-verified on macOS 26.4: 500 concurrent curl through the proxy,
single ProxyPin process throughout, 9099 freed immediately on exit.

lib/network/util/process_info_macos.dart

@@ -93,6 +93,13 @@ class MacosProcessInfo {
   /// are spawned. Typical cost on a desktop with ~500 processes and ~10k
   /// total fds is a few milliseconds.
   static int? findPidByLocalTcpPort(int localPort) {
+    // The insi_lport read below assumes a little-endian host: `(int)htons(port)`
+    // stores the network-byte-order 16-bit port in the low two bytes of the
+    // int32 field. All shipping macOS hardware (x86_64 / arm64) is
+    // little-endian; this assert exists to fail loudly rather than return
+    // wrong port values if that ever changes. Stripped in release builds.
+    assert(Endian.host == Endian.little, 'libproc parsing requires little-endian host');
+
     final pidBufSize = _procListPids(_kProcAllPids, 0, nullptr, 0);
     if (pidBufSize <= 0) return null;