test: resolve packed files from the npm root

[ScriptedAlchemy]

Follow-up to #656 after merged-main CI exposed stale prepack fixture paths.

• create simulated vendored dependencies under the generated npm root
• name declaration files relative to that root
• exercise optional dependency evidence through the package inventory directly

Local gate:

• PASS — pnpm build
• PASS — pnpm exec rstest --config rstest.integration.config.ts packages/agent-bundle/tests/prepack.test.ts (25 passed)
• PASS — pnpm typecheck && pnpm lint && pnpm test:unit

Deslop: GPT-5.6 Sol, 1 edit (removed the obsolete authored files mutation instead of adapting it).

Self-review: Claude Fable 5.1 Thinking High found no concrete merge risks. The warning-only public-prepack return path is no longer covered by this fixture; accepted because this hotfix directly exercises package-inventory evidence and the shared error gate remains covered elsewhere. Existing non-finally vendor cleanup is unchanged in structure.

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