Read url_helper_name and url_options from what their writers set

[namolnad]

url_helper_name and url_options are declared with mattr_writer, which stores the value in a class variable (@@url_options). Their readers memoize an instance variable (@url_options ||= detect_url_options) and never look at it. So the engine's config.solid_errors.each { SolidErrors.public_send("#{name}=", value) } writes a value that nothing reads, and the email falls back to route detection.

That shows up as an error email with no link whenever detection can't find the route. Two common cases:

• The engine is mounted under a name that doesn't end in solid_errors, e.g. mount SolidErrors::Engine, at: "errors", as: "admin_errors". Detection falls back to :solid_errors, occurrence_url raises NoMethodError, the helper rescues it, and the link is dropped.
• The dashboard lives on a different host from ActionMailer's default_url_options, so the link needs url_options: {host: ...}.

config.solid_errors.url_options = {host: "admin.example.com", protocol: "https"}
SolidErrors.url_options # => {} before this change

The fix makes the readers memoize into the same class variables the writers set. Explicit config wins, and detection still fills in when nothing is configured.

Added a mailer test that sets url_options and checks the link. It fails before the change and passes after.