Added nullability attributes to the API

[jstedfast]

Fixes issue #1972

[Copilot]

Pull request overview

This pull request adds nullable reference type annotations to MailKit's public API to address issue #1972. The changes enable C# 8.0+ nullable reference types across the codebase, providing compile-time null safety.

Changes:

• Enabled nullable reference types in MailKit.csproj by setting <Nullable>enable</Nullable> and upgrading to LangVersion 12
• Added nullable annotations (?) to reference type parameters, properties, and return types throughout the API
• Updated unit tests to use proper constructors instead of object initializers for BodyPart types
• Removed obsolete parameterless constructors for event args classes
• Added proper null checks and null-forgiving operators where needed

Reviewed changes

Copilot reviewed 126 out of 126 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
MailKit/MailKit.csproj	Enabled nullable reference types and upgraded C# language version to 12
MailKit/Security/*.cs	Added nullable annotations to SASL mechanism classes and SSL exception handling
MailKit/Net/**/*.cs	Added nullable annotations to networking, IMAP, POP3, and SMTP client classes
MailKit/*.cs	Added nullable annotations to core MailKit types like MessageSummary, Envelope, BodyPart classes
UnitTests/*.cs	Updated tests to use proper constructors for BodyPart types and adjusted assertions for non-null collections

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[coveralls]

coverage: 0.0% (-92.9%) from 92.887%
when pulling adfebe6 on nullability-api
into a2766da on master.