Cover Express not-found delegation

[aryansk]

Summary

• Add focused @fedify/express regression coverage for not-found delegation.
• Assert Express next() is invoked when Fedify calls onNotFound.

Fixes #857

Validation

• node --experimental-transform-types --test packages/express/src/index.test.ts
• deno fmt --check packages/express/src/index.test.ts
• deno lint --no-config packages/express/src/index.test.ts

The repository-wide mise run check-each express was not completed in this checkout because the pinned clean-checkout bootstrap attempted uncached monorepo downloads; the focused regression, formatting, and lint checks pass.

AI disclosure: OpenAI Codex (GPT-5) assisted with the implementation and validation of this issue-scoped test.

[netlify]

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