Preserve error response body when error field is absent

[FU-max-boop]

Summary

• fall back to the full JSON error response when a response body does not contain an error field
• preserve the existing behavior of preferring the nested error object when present
• add regression coverage for compatible APIs that return bodies like { detail: ... }

Fixes #1734

Tests

• npx -y -p [email protected] -p @swc/[email protected] -p @swc/[email protected] jest tests/error.test.ts --runInBand --config '{...repo-equivalent targeted config...}'
• npx -y [email protected] --check src/core/error.ts tests/error.test.ts

Note: yarn install --frozen-lockfile was blocked locally by a timeout downloading the repository's tsc-multi GitHub release tarball, so I ran the focused Jest test through npx with the same SWC transform/module mapping instead.