Warn when -J falls back from Content-Disposition

[ryanfowler]

Summary

• Emit a warning when --remote-header-name is requested but no usable Content-Disposition filename is available
• Keep the fallback behavior intact, using the URL path or host filename as before
• Update docs and add unit/integration coverage for missing and invalid header cases

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features --test cli --test formatting --test grpc --test http --test network --test terminal --test update --test websocket -- --test-threads=1
• Focused unit and HTTP integration tests for -O -J fallback behavior