Escape quotes in MailAddress display name when encoding SMTP headers

[Copilot]

MailAddress.Encode — the path SmtpClient uses to write From/To/Cc/etc. headers — wraps the display name in "…" but does not escape embedded ", producing an invalid RFC 5322 quoted-string and corrupting the header when a DisplayName like Henry "The Fonz" Winkler is used. MailAddress.ToString() already escapes quotes; only the SMTP send path was missed when quote support was added in #36752.

Changes

• MailAddress.Encode (ASCII / allowUnicode=true branch): escape embedded " as \" via .Replace("\"", "\\\""), matching ToString(). The Q-encoded (non-ASCII) branch is unaffected since encoded-words don't use quoted-string syntax.
• Tests: add regression tests in MailAddressEncodeTest for both MailAddress.Encode and MailAddressCollection.Encode, covering ASCII and Unicode-allowed modes.

Before vs. after for new MailAddress("test@example.com", "Henry \"The Fonz\" Winkler").Encode(0, false):

- "Henry "The Fonz" Winkler" <test@example.com>     // invalid quoted-string
+ "Henry \"The Fonz\" Winkler" <test@example.com>   // RFC 5322-compliant

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