Simplify URL parsing detection in metadata

[ryanfowler]

Summary

• Replace the raw contains("://") check with explicit authority-scheme detection.
• Keep schemeless host parsing as a separate path so example.com, localhost:3000, and IP literals still work.
• Add regression tests for explicit schemes, schemeless defaults, invalid inputs, and :// appearing in paths or queries.

Testing

• Unit tests added around normalize_url and the explicit-scheme detector.
• Local validation passed with cargo fmt, cargo clippy --locked --all-targets --all-features -- -D warnings, cargo test --all-features, and the serialized integration test suite.