chore: Email attachment testcase (Fixes #812)

[arky]

Surface email attachments feature in mail executor and update docs.

[arky]

@yohamta All lint tests now work correctly!

[yottahmd]

Looking good to me! Thanks for implementing the attachment functionality and updating the document!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.93%. Comparing base (39f0ce8) to head (ec8c986).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #814      +/-   ##
==========================================
- Coverage   56.06%   55.93%   -0.14%     
==========================================
  Files          73       73              
  Lines        7921     7920       -1     
==========================================
- Hits         4441     4430      -11     
- Misses       3102     3110       +8     
- Partials      378      380       +2     

Files with missing lines	Coverage Δ
internal/digraph/executor/mail.go	32.75% <ø> (ø)

... and 16 files with indirect coverage changes

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