feat(rust-sdk): return URL or fd from tunnel endpoint

[G4614]

Return a typed tunnel endpoint that exposes a public URL for remote boxes or an owned FD for local boxes, while connect consumes the prepared local connection.

Test plan:

• BOXLITE_DEPS_STUB=1 cargo clippy -p boxlite --all-targets --all-features -- -D warnings
• BOXLITE_DEPS_STUB=1 cargo test -p boxlite --lib litebox::network::tests -- --nocapture

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