Reject domains longer than 253 characters

[eeshsaxena]

Fixes #413.

domain() validated each label ({0,61} + boundary chars, i.e. ≤ 63 characters per label) but never checked the total length. A name made of several valid-length labels could therefore exceed the RFC 1035 limit and still be accepted, and hostname() — which delegates to domain() — inherited the same gap:

>>> long_name = ".".join(["a" * 49] * 5) + ".aaaaa"   # 255 chars, every label <= 63
>>> validators.hostname(long_name)   # was True
>>> validators.domain(long_name)     # was True

The textual representation of a domain name is limited to 253 characters (RFC 1035, one octet of the 255-octet wire limit is the root label). This adds that check to domain(), which fixes both domain() and hostname().

• domain()/hostname() now reject the 255-character example above.
• A name of exactly 253 characters is still accepted (boundary covered by the new test).
• Existing valid domains are unaffected.

tests/test_domain.py + tests/test_hostname.py: 88 passed. ruff check / ruff format --check clean.