Prevent corrupted SMTP headers when MailAddress.DisplayName contains CR/LF

[Copilot]

MailAddress accepted display names containing \r/\n and SmtpClient embedded them verbatim into From:/To: headers, breaking header structure and SMTP framing. Typical source: a trailing newline left over from parsing upstream input.

Changes in MailAddress.TryParse (src/libraries/System.Net.Mail/src/System/Net/Mail/MailAddress.cs):

• Caller-supplied display name: reject CR/LF by throwing FormatException, matching the existing convention for MailMessage.Subject (MailBnfHelper.HasCROrLF → MailSubjectInvalidFormat). TryCreate returns false.
• Display name parsed out of the address string: unfold per RFC 5322 §3.2.4 by stripping CR/LF from the stored value. This preserves the previously-accepted folded input "display \r\n name" <"folding"@domain.com> while ensuring re-encoding produces a clean header.
• New StripCRLF helper (guarded by an HasCROrLF pre-check, so allocation only occurs when needed).

// Now throws FormatException instead of producing a broken From: header
new MailAddress("user@example.com", "Display Name\r\n");

Test coverage added in MailAddressDisplayNameTest.MailAddress_Ctor_DisplayNameContainsCRLF_Throws for CR, LF, and CRLF in leading/middle/trailing positions and inside bounding quotes, against both the constructor and TryCreate.

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