Fix rust-analyzer proc-macro merge for k8-opt-exec paths (#4140)

Written by AI, reviewed by human.

Fixes #4139.

## Summary

`consolidate_crate_specs` prefers proc-macro dylib paths that look like
exec outputs. The previous marker matched paths with `-opt-exec-`, but
Bazel can also emit exec paths like `bazel-out/k8-opt-exec/bin/...`.

This updates the marker to match both forms and adds a regression test
for the unsuffixed `k8-opt-exec/bin` path shape.

## Testing

```sh
bazel test --cache_test_results=no //tools/rust_analyzer:gen_rust_project_lib_test
```