fix: finish spans when decorated methods throw

[seandearnaley]

Summary

• finish the active span before propagating synchronous errors from decorated methods
• keep asynchronous rejections tagged, finalized, and rethrown
• assert that both paths preserve the original error and finish the span exactly once

Problem

recordException() previously tagged and immediately rethrew the error. In the synchronous catch path, that made the following span.finish() unreachable and left the span open. Promise paths happened to finish through .finally(), but their error propagation depended on the helper throwing.

This change makes recordException() responsible only for tagging. Each caller now explicitly propagates the original error, after the synchronous path has finalized its span; asynchronous paths continue to finalize through .finally().

This is a lifecycle bug fix for the package's existing dd-trace ^5 support. It intentionally does not change the peer range, development dependency, Node.js CI version, or lockfile.

Verification

• npm test -- --runInBand — 2 suites, 20 tests passed
• npm run build
• ESLINT_USE_FLAT_CONFIG=false npx eslint src/decorator.injector.ts src/decorator.injector.spec.ts --no-fix
• repository-wide lint was also checked; it remains blocked by 41 pre-existing formatting/unused-import findings in src/datadog-trace.module.ts and src/datadog-trace.module.spec.ts