Test loader paths with loaders/ prefix in error assertions
- Add loaders/error-loader path assertion for Turbopack (which preserves
loader paths in stack traces)
- Use loaders/ prefix for promise-error and timeout-error assertions to
verify the loader path is actually present, not just matching the name
in the error message
- Document that string-error case loses loader path in both modes:
webpack strips it via format-webpack-messages, turbopack only shows
internal frames for string throws
- Document that webpack strips loader path for Error throws via
format-webpack-messages filtering "Module build failed (from ...)" lines
https://claude.ai/code/session_01XnHv9KVqYRDLDti1q6vni4
