fix: preserve original email string for quoted local parts with backslash escapes

[gaoflow]

Bug

When validate_email is called with a quoted local part that contains backslash-escaped characters — for example "test\"foo"@example.com — the original field on the returned ValidatedEmail object is built incorrectly.

The code reconstructs ret.original by re-quoting the already-unescaped local_part. Since split_email() returns the local part with escapes removed (i.e. test"foo), re-wrapping it in quotes produces "test"foo"@example.com — an invalid, malformed string that is also one character shorter than the actual input.

from email_validator import validate_email

email = '"test\\"foo"@example.com'
r = validate_email(email, check_deliverability=False, allow_quoted_local=True)

# Before fix:
# r.original == '"test"foo"@example.com'  ← wrong (invalid, missing backslash)
# r.normalized == '"test\\"foo"@example.com'  ← correct

# After fix:
# r.original == '"test\\"foo"@example.com'  ← correct (same as input)
# r.normalized == '"test\\"foo"@example.com'  ← correct

The documented contract (ValidatedEmail.original: "The email address that was passed to validate_email.") is violated. Additionally, the incorrect shorter string is used for email length checks (validate_email_length), which could allow emails that are actually over the 254-byte limit to pass when they contain backslash-escaped characters in a quoted local part.

Fix

When no display name is present, use the original email argument directly — no reconstruction needed. When a display name is present (e.g. "John" <addr@domain>), extract the address from inside the angle brackets to preserve the existing behavior of stripping the display name for email length checks.

All 288 existing syntax tests pass.

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This pull request was prepared with the assistance of AI, under my direction and review.