fix: Remove `turborepo-engine`'s `unwrap()` usage (#12906)
## Why
`turborepo-engine` still carried a crate-level `.unwrap()` allowance
even though implementation usage was limited. Removing it brings the
crate under the workspace panic-hardening policy for unwraps.
Closes TURBO-5645
## What
Removes implementation unwraps from task inheritance validation and
diagnostic URL generation, then narrows the engine lint allowance to the
separate expect cleanup.
## How
- `cargo fmt -p turborepo-engine`
- `cargo test -p turborepo-engine`
- `cargo clippy -p turborepo-engine --all-targets -- -D warnings`
- Pre-push hook passed with format, check:toml, and Rust checks
