fix: relay raw SMTP bytes without decoding

send_message decoded the message body with utf-8/errors="replace",
corrupting 8-bit content before forwarding. Pass the raw bytes straight
to smtp.sendmail so the message is relayed unchanged.

Fixes #4

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_proxy_server.py

@@ -126,14 +126,17 @@ def test_smtp_proxy_handler_forwards_message_over_ssl(monkeypatch):
     Settings.BACKEND_SMTP_USER = "smtp-user"
     Settings.BACKEND_SMTP_PASS = "smtp-pass"
 
+    # Include an 8-bit byte that would be mangled by a utf-8 decode.
+    raw = b"Subject: Test\r\n\r\nBody \x80\r\n"
     handler = SMTPProxyHandler()
-    handler.send_message("from@example.com", ["to@example.com"], b"Subject: Test\r\n\r\nBody\r\n")
+    handler.send_message("from@example.com", ["to@example.com"], raw)
 
     assert captured["instance"].host == Settings.BACKEND_SMTP_HOST
     assert captured["instance"].logged_in is True
     assert captured["instance"].sent[0] == "from@example.com"
     assert captured["instance"].sent[1] == ["to@example.com"]
-    assert "Subject: Test" in captured["instance"].sent[2]
+    # Raw bytes must be forwarded unchanged, not decoded.
+    assert captured["instance"].sent[2] == raw
 
     Settings.BACKEND_SMTP_USE_SSL = previous_ssl
     Settings.BACKEND_SMTP_USER = previous_user