fix(turbopack): handle cyclic or rootless nodes in ModuleGraphImportTracer

[agentHits]

What?

In turbopack/crates/turbopack-core/src/module_graph/mod.rs, replaces unreachable!("there must be a path to a root") with a graceful fallback to vec![module_idx] when astar cannot find a root in the reversed graph.

Why?

During standalone static prerendering with complex CSS / SCSS import trees, when Turbopack formats an issue diagnostic via ModuleGraphImportTracer::get_traces, cyclic imports or orphaned asset nodes can cause astar on reversed_graph to return None (since no node satisfies neighbors(n).next().is_none()).

Previously, this triggered an unexpected Rust panic that killed the entire build worker:

thread 'tokio-rt-worker' panicked at turbopack/crates/turbopack-core/src/module_graph/mod.rs:750:25:
internal error: entered unreachable code: there must be a path to a root

How?

Instead of panicking with unreachable!, Turbopack now falls back to returning the single module_idx so issue reporting can proceed without terminating the process:

let path = match petgraph::algo::astar(
    &reversed_graph,
    module_idx,
    |n| reversed_graph.neighbors(n).next().is_none(),
    |e| match e.weight().chunking_type {
        ChunkingType::Parallel { .. } => 0,
        _ => 1,
    },
    |_| 0,
) {
    Some((_, path)) => path,
    None => vec![module_idx],
};

[agentHits]

Fixes #98205

Linked Issue: #98205
Reproduction & Benchmark: https://github.com/agentHits/next-prerender-workstore-reproduction