Potential issue: a leading ":" in a URL string is silently eaten, breaking round-tripping

[Sun-Wukhan]

Found while property-based testing URL parse/print round-trips.

u = httpx.URL(":http://e/")
str(u)                      # "http://e/"
u.is_absolute_url           # False — parsed as a relative URL
httpx.URL(str(u)) == u      # False — the same string re-parses as absolute, host "e"

httpx.URL(":foo")           # accepted, colon silently dropped -> path "foo"

The URL regex's scheme group can match empty, so a leading ':' is consumed as a scheme delimiter with an empty scheme. That also bypasses validate_path's own rule — "Relative URLs cannot have a path starting with ':'" — which is enforced for component-wise construction but not for string parsing.

One wrinkle: httpx.URL("://example.com") is pinned by an existing test (test_url_no_scheme) as a deliberately supported scheme-less authority form, so a fix has to reject ":foo" / ":http://..." while keeping "://example.com" working.

The fix is ready on a fork branch: https://github.com/Sun-Wukhan/httpx/tree/fix/url-leading-colon (the full tests/models suite passes with it). Raising it here first since PR creation on this repo is limited to collaborators. Happy either way — mainly wanted to check this isn't intentional leniency.