Load Logger when configuring custom logger delivery

[OskarEichler]

Summary

Require logger when the LoggerDelivery implementation loads, rather than only when constructing its default logger. A supplied logger currently bypasses that require and raises NameError while resolving the default or named severity in a fresh process.

Reproduction

Run in a fresh process against the gem:

require 'mail'
custom = Object.new
custom.define_singleton_method(:log) { |level, &block| [level, block.call] }
p Mail::LoggerDelivery.new(logger: custom).severity # => 1; previously NameError

Verification

• Existing bundle exec rake spec: 1,837 examples, zero failures, four existing pending, both before and after this isolated patch, on Ruby 4.0.6 via rbenv.
• External focused checks: 15 checks; 12 baseline failing expectations → zero.
• Fresh process without a preloaded Logger; nil, symbol, string and integer severity; custom delivery receives the expected severity and encoded message. Existing logger specs preload logger and therefore miss the initial load-order failure. Existing Add Logger delivery method #856 introduction reviewed.
• Targeted syntax and git diff --check pass. Supplementary Lint output retains the same 275 existing findings by file/cop/message.

Limitations and contribution policy

CONTRIBUTING.md and relevant existing specs were reviewed; a changelog entry is included. The requesting project's explicit no-new-tests policy prevents adding or modifying repository specs, so this PR supplies an inline reproduction and runs the existing suite plus external checks. This differs from upstream's request for a regression spec and may need maintainer follow-up. No claim of coverage completeness, other Ruby engines/versions, production throughput, or live delivery. Verification uses an external bundle with the consumer's patched network/mini_mime dependencies; benchmark is supplied externally for Ruby 4 (existing #1655/#1666), without changing the gem dependencies or CI.

Breaking changes

No intended breaking changes. Logger is already a runtime dependency and remains lazy with respect to require "mail"; it loads when LoggerDelivery is first used.