Form the correct MIME structure when there's an alternative body and attachment(s)

[jennybc]

Fixes #202

[jennybc]

Good manual tests (but, yes, actual tests have been added as well):

# Pick one, depending on your situation
#library(gmailr)
devtools::load_all()

# Do you need to authenticate?
# Not necessary when I do `load_all()` as it uses existing token
# gm_auth()

# Test 1: Text + HTML + PDF attachment (send as draft)
# This was one of the original reported scenarios
message1 <- gm_mime() |>
  gm_to("jenny+gmailr-tests@posit.co") |>
  gm_subject("Test 1: Text+HTML+PDF - Issue #202") |>
  gm_text_body("This is the plain text body.") |>
  gm_html_body(
    "<p>This is the <strong>HTML</strong> body.</p>"
  ) |>
  gm_attach_file(R.home('doc/html/Rlogo.pdf'))

# Create draft (don't send yet - look at the draft first)
draft1 <- gm_create_draft(message1)

# Now send it and check what you received
gm_send_draft(draft1)

# Test 2: Text + HTML + TXT attachment (send directly)
# This was the other main scenario from the issue
message2 <- gm_mime() |>
  gm_to("jenny+gmailr-tests@posit.co") |>
  gm_subject("Test 2: Text+HTML+TXT - Issue #202") |>
  gm_text_body("Plain text body for text attachment test.") |>
  gm_html_body("<p><em>HTML body</em> for text attachment test.</p>") |>
  gm_attach_file(system.file("DESCRIPTION"), type = "text/plain")

# Send directly
gm_send_message(message2)

# Test 3: Multiple attachments (send as draft)
# Verify the fix works with multiple attachments
message3 <- gm_mime() |>
  gm_to("jenny+gmailr-tests@posit.co") |>
  gm_subject("Test 3: Text+HTML+Multiple attachments - Issue #202") |>
  gm_text_body("Testing with multiple attachments (plain text).") |>
  gm_html_body(
    "<p>Testing with <strong>multiple attachments</strong> (HTML).</p>"
  ) |>
  gm_attach_file(R.home('doc/html/Rlogo.pdf')) |>
  gm_attach_file(R.home('doc/html/logo.jpg'))

draft3 <- gm_create_draft(message3)
gm_send_draft(draft3)