fix: correct duplicate message reporting in Go implementation

- Add DocumentSkippedError custom error type to distinguish between skipped and stored documents
- Fix counter bug where skipped messages were incorrectly reported as "stored"
- Enhance status reporting to show "X skipped as duplicates" for better visibility
- Fix Rust implementation binary attachment handling to support all file types (images, PDFs, etc.)
- Update test scripts to use correct binary names (mail2couch-go, mail2couch-rs)
- Add comprehensive test configurations for implementation comparison

Before: "Summary: Processed 30 messages, stored 30 new messages" (misleading when all were duplicates)
After: "Summary: Processed 30 messages, stored 0 new messages" with detailed "Stored 0/30 messages from INBOX (30 skipped as duplicates)"

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

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

test-config-go.json

@@ -0,0 +1,24 @@
+{
+  "couchDb": {
+    "url": "http://localhost:5984",
+    "user": "admin",
+    "password": "password"
+  },
+  "mailSources": [
+    {
+      "name": "go_test_account",
+      "enabled": true,
+      "protocol": "imap",
+      "host": "localhost",
+      "port": 3143,
+      "user": "testuser1",
+      "password": "password123",
+      "mode": "archive",
+      "folderFilter": {
+        "include": ["INBOX"],
+        "exclude": []
+      },
+      "messageFilter": {}
+    }
+  ]
+}