refactor: modularize IMAP server with dual login, STARTTLS, and test suite (#859)

refactor: modularize IMAP server with fixes and E2E tests

- Modularize IMAP server into imap_server, imap_mailbox, imap_message,
  imap_http_client, parse_email, config, models
- Support dual login: JWT token and address+password via backend
- Add STARTTLS support with configurable TLS cert/key
- Fix FETCH/STORE returning UID instead of sequence number (RFC 3501)
- Implement IMessageFile.open() for correct BODY[] raw MIME delivery
- Add UIDNEXT to SELECT response via _cbSelectWork override
- Use per-restart UIDVALIDITY to force client resync
- Pass raw MIME to SimpleMessage for accurate RFC822.SIZE
- Fix SENT mailbox returning empty source
- Handle CREATE command gracefully for Thunderbird compatibility
- Add IMAP E2E tests: auth, LIST, SELECT, STATUS, FETCH, SEARCH,
  STORE, UID FETCH, BODY[] integrity, size, seq numbers, SENT mailbox
- Add SMTP E2E tests using nodemailer: send plain/HTML, auth failure,
  sendbox verification
- Add sendTestMail helper using admin/send_mail

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

smtp_proxy_server/parse_email.py

@@ -19,7 +19,15 @@ def get_email_model(msg: Message):
         get_email_model(subpart)
         for subpart in msg.get_payload()
     ] if msg.is_multipart() else []
-    body = "" if msg.is_multipart() else msg._payload
+    if msg.is_multipart():
+        body = ""
+    else:
+        raw_body = msg.get_payload(decode=True) or b""
+        charset = msg.get_content_charset() or "utf-8"
+        try:
+            body = raw_body.decode(charset, errors="replace")
+        except LookupError:
+            body = raw_body.decode("utf-8", errors="replace")
     return EmailModel(
         headers={k: v for k, v in msg.items()},
         body=body,
@@ -44,7 +52,12 @@ def parse_email(raw: str) -> EmailModel:
         )
 
 
-def generate_email_model(item: dict) -> EmailModel:
+def generate_email_model(item: dict) -> tuple[EmailModel, str]:
+    """Build an EmailModel from a sendbox item.
+
+    Returns (EmailModel, raw_mime_string) so callers can pass the
+    synthesised MIME to SimpleMessage for correct BODY[] responses.
+    """
     email_json = json.loads(item["raw"])
     message = MIMEMultipart()
     if email_json.get("version") == "v2":
@@ -66,4 +79,5 @@ def generate_email_model(item: dict) -> EmailModel:
     message["Date"] = datetime.datetime.strptime(
         item["created_at"], "%Y-%m-%d %H:%M:%S"
     ).strftime("%a, %d %b %Y %H:%M:%S +0000")
-    return parse_email(message.as_string())
+    raw_mime = message.as_string()
+    return parse_email(raw_mime), raw_mime