feat: fix attachment stub issues in Rust implementation

- Removed attachment metadata from initial document storage
- Attachments are now stored separately using CouchDB native attachment API
- This matches the Go implementation approach and resolves CouchDB validation errors
- All messages with attachments now store successfully

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

rust/config-test-rust-single.json

@@ -0,0 +1,24 @@
+{
+  "couchDb": {
+    "url": "http://localhost:5984",
+    "user": "admin",
+    "password": "password"
+  },
+  "mailSources": [
+    {
+      "name": "Rust Wildcard All Folders Test",
+      "enabled": true,
+      "protocol": "imap",
+      "host": "localhost",
+      "port": 3143,
+      "user": "testuser1",
+      "password": "password123",
+      "mode": "archive",
+      "folderFilter": {
+        "include": ["*"],
+        "exclude": ["Drafts", "Trash"]
+      },
+      "messageFilter": {}
+    }
+  ]
+}