Add required environment marker example to hint (#16244)
## Summary
fixes issue #15938
- show platform wheel hint with a concrete
`tool.uv.required-environments` example so users know how to configure
compatibility
- add `WheelTagHint::suggest_environment_marker` to pick a sensible
environment marker based on the available wheel tags
- update the `sync_required_environment_hint` integration snapshot to
expect the new multi-line hint
## Test Plan
cargo test --package uv --test it --
sync::sync_required_environment_hint
