Fix from-curl proto schemeless URL handling

[ryanfowler]

Summary

Fix --from-curl --proto handling for schemeless URLs that contain :// later in the path, query, or fragment.

The previous check used a loose split_once("://"), which could mistake text after the authority position for an explicit URL scheme. This meant a command such as curl --proto '=http' example.com/path://still could bypass the intended schemeless defaulting path and later normalize as HTTPS.

This change reuses the HTTP URL authority-scheme detector so --from-curl --proto only treats a URL as explicitly schemed when the scheme appears before /, ?, or # and is followed by //. Regression coverage now covers :// in the path, query, and fragment.

Validation

• cargo fmt
• cargo fmt --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all-features --lib --bins
• cargo test --locked --all-features from_curl_proto
• cargo test --locked --all-features authority_scheme_detection_requires_valid_leading_scheme