fix: prevent HTTP/3 cache symlink traversal

[ryanfowler]

Summary

• restrict HTTP/3 cache pruning to the expected root/shard/file layout
• skip symlinked roots, directories, and files
• add coverage for directory symlinks, file symlinks, and symlink cycles

Validation

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all-features --lib http3_cache::tests::global_pruning_skips_directory_and_file_symlinks_and_cycles