Sent-mail matchers raise when expected fields are absent

## Problem

Attachment and sent-mail matchers dereference optional message fields without checking for absence. Matching messages that omit those fields can raise instead of returning a normal mismatch.

Prepared commit `9b8e6494e3a89be1f1da6cdd1d5f8ddd94bc0b0d` makes the optional-field comparisons nil-safe without changing populated-field matching.

External matcher models cover absent/present fields, regex/string expectations and attachments. The unchanged 1,837-example suite passes on Ruby 4.0.6 and 3.2.11; package and Action Mailer integration pass.

CONTRIBUTING requires regression specs, so this report does not open a source-only PR or modify tests.

Start with the sent-mail and attachment matcher implementations and the external matcher models covering absent and present fields. Run the relevant matcher examples first, then check package and Action Mailer integration. Done means absent optional fields produce a normal mismatch, populated-field matching remains unchanged, and regression specs satisfy CONTRIBUTING.

Written by the indexing model from the issue text.