URL.replace(hostname=...) creates an invalid authority for unbracketed IPv6

[Str0k]

On main f03f65c, passing an unbracketed IPv6 address as hostname to URL.replace() produces an authority that cannot be parsed correctly.

from starlette.datastructures import URL

url = URL("https://example.org:8443/path?a=1#anchor")
result = url.replace(hostname="::1")

print(str(result))     # https://::1:8443/path?a=1#anchor
print(result.hostname)  # None
print(result.port)      # ValueError: Port could not be cast to integer value as ':1:8443'

Expected: https://[::1]:8443/path?a=1#anchor, with hostname ::1 and port 8443. This also affects round-tripping a hostname obtained from another URL.

I checked the IPv6 and URL.replace issues, PRs and discussions. #1965 preserves an existing bracketed hostname when other components change; this case supplies a new hostname explicitly.

A small patch brackets a supplied colon-containing hostname unless it is already bracketed, matching the existing scope-construction approach:
patch and regression tests.

Validation on Windows / Python 3.11.15:

• The 12 new parameterized cases fail before the change and pass after it.
• tests/test_datastructures.py tests/test_requests.py tests/test_routing.py: 249 passed.
• Ruff check/format and mypy pass for both changed files.

The cases cover port omission, port zero, a normal port, mapped/scoped IPv6, already-bracketed input and hostname round trips.

Would accepting unbracketed IPv6 hostnames here be the intended behavior? Opening this discussion before a PR as requested by the contribution guide.

Prepared with Codex assistance.