Return an error when Cargo.toml is missing

[Kuhai9801]

Summary

find_manifest_dir currently panics if it walks up to the filesystem root without finding a Cargo.toml, because it unwraps Path::parent().

This changes that path to return a normal eyre error instead, matching the function's Result return type.

Testing

• cargo test -p cargo-openvm --lib find_manifest_dir --no-default-features
• cargo check -p cargo-openvm --no-default-features
• git diff --check
• rustfmt --check crates/cli/src/util.rs