🪲 BUG-#68: Avoid mutating caller EmailMessage when adding From header

[FernandoCelmer]

Summary

• Sender.send_message now deepcopy's the caller's EmailMessage before injecting the From header when missing
• Prevents duplicate From: headers on retry and silent mutation of caller-owned objects
• Tests cover both the non-mutation invariant and the two-call retry path

Closes #68

Test plan

• pytest tests/clients/smtp/test_send.py — 10 passed

[FernandoCelmer]

🔍 Code Review

Code issues found: 0

copy.deepcopy before injecting From is the minimal correct fix. Both tests are meaningful:

1. Caller's message is left untouched.
2. Two consecutive sends produce one From header on the wire — no header duplication on retry.

One minor non-blocking observation for the future (not required to merge): deepcopy of an EmailMessage with large attachments is O(payload size). Since the copy only happens when From is missing, this is fine in practice — but if you ever start mutating other headers, consider email.message_from_bytes(msg.as_bytes()) or a shallow rebuild instead.

Closes #68.