Split recipient strings without breaking a quoted display name

[pcbeingused333]

The bug

Courrier::Email::Address.with_name quotes any display name that contains a special character, so with_name("jane@example.com", "Doe, Jane") returns:

"Doe, Jane" <jane@example.com>

Every provider then feeds to / cc / bcc / from / reply_to through Providers::Base#address_list, which does value.to_s.split(","). That tears the quoted name in two:

'"Doe, Jane" <jane@example.com>'.split(",").map(&:strip)
# => ["\"Doe", "Jane\" <jane@example.com>"]

So the recipient goes out as two malformed addresses. address_line (Mailgun, MailPace, Postmark, and SparkPost's CC header) rejoins the pieces with ", ", which hides the break for a single recipient but still rewrites any name whose comma isn't already followed by exactly one space ("Doe,Jane" → "Doe, Jane").

The address test suite already pins with_name producing "Doe, John" <...>, so the two halves of the library disagree.

The fix

split_addresses now splits only on commas that are outside a double-quoted display name ("(?:\\.|[^"\\])*"), so a quoted name survives intact and a plain a@x, b@y list still splits as before. Blank input (nil, "", " ", ",") still returns nil.

Three providers still hand-rolled their own recipient parsing and had the same class of bug; they now go through address_list like everyone else:

• Smtp2go, Lettermint — @options.to.to_s.split(","), same comma-in-name break. The helper also drops their empty "" entries the way Refactored into reusable email addressess list helper #61 did for the others.
• SES — Array(@options.to), which wraps a comma-separated string in a one-element list instead of splitting it, so several recipients went out as one malformed address (the shape of Mailjet sends multiple recipients as a single malformed address #59) and an empty cc became [""].

Tests

• test/courrier/providers/base_test.rb — address_list / address_line directly: quoted comma, quoted comma among several addresses, escaped quote, blank input.
• New smtp2go_test.rb, lettermint_test.rb, ses_test.rb (none had a test file) — full body, quoted-name recipient, comma-string splitting, empty-field omission, registry wiring.
• Added the quoted-name case to brevo_test (address_list path) and mailgun_test (address_line path).

11 of the new assertions fail on main and pass with the fix. bundle exec rake and bundle exec standardrb are green (181 runs, 0 failures).

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