Fix subscriber requests that address the wrong member via the URL

[pcbeingused333]

Four subscriber providers build a request URL from the raw email and end up addressing the wrong member.

Buttondown, Mailerlite, Beehiiv — not URL-encoded

request(:delete, "#{ENDPOINT_URL}/#{email}")                          # Buttondown, Mailerlite
request(:get, "#{ENDPOINT_URL}/#{publication_id}/subscriptions?email=#{email}")  # Beehiiv

A Gmail-style +tag address is mangled — + decodes to a space in a query string and is ambiguous in a path:

Courrier::Subscriber::Buttondown.new(api_key: k).destroy("user+news@gmail.com")
# DELETE .../subscribers/user+news@gmail.com  -> wrong subscriber or 404

Subscriber::Brevo#destroy already guards this with URI.encode_www_form_component; these three now do the same.

Mailchimp — wrong identifier entirely

Mailchimp's List Members API keys a member by the MD5 hash of the lowercased email, so DELETE .../members/#{email} never matches and always 404s. It now hashes the address before building the path.

Tests

New test/courrier/subscriber/url_encoding_test.rb: a +tag address reaches Buttondown / Mailerlite / Beehiiv URLs as subscriber%2Btag%40example.com, and Mailchimp addresses Subscriber@Example.com by Digest::MD5.hexdigest("subscriber@example.com"). All four fail on main. bundle exec rake and bundle exec standardrb are green (163 runs, 0 failures).

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