refactor: preserve original part order in multipart/related handling …

[Aravinda-HWK]

📌 Description

This PR is to resolve the part number problem for a normal mail client, like Gmail

• Closes [BUG] Email parts are not fetched from the server #208

---

🔍 Changes Made

• Initially, part numbers were like 1-n numbers, but now it changed to for each parent, its children's parts start counting from 1.
• When fetching the mail from the database, it should now change based on the database id not the part number.( Changed this)
• Remove unused sortMultipartRelatedChildrenand prepareMultipartRelated because when they are stored in the database, they are sorted, and preparation is also the same.
• Check if we have a single root multipart container. If so, skip it and use its children as IMAP top-level parts. (Updated in the fetch.go)

✅ Checklist (Email System)

• Core IMAP commands tested (LOGIN, CAPABILITY, LIST, SELECT, FETCH, LOGOUT).
• Authentication is tested.
• Docker build & run validated.
• Configuration loading verified for default and custom paths.
• Persistent storage with Docker volume verified.
• Error handling and logging verified
• Documentation updated (README, config samples).

---

🧪 Testing Instructions

To test the server, use the instructions in the README in the test directory.

---

📷 Screenshots / Logs (if applicable)

---

⚠️ Notes for Reviewers

Run the email server, connect to a few different types of MUA, and check the email by sending different type of emails.

[gemini-code-assist]

Important

Installation incomplete: to start using Gemini Code Assist, please ask the organization owner(s) to visit the Gemini Code Assist Admin Console and sign the Terms of Services.

[gemini-code-assist]

Important

Installation incomplete: to start using Gemini Code Assist, please ask the organization owner(s) to visit the Gemini Code Assist Admin Console and sign the Terms of Services.