test: replace stale serve-static source-text assertion

[flyingrobots]

Problem

test/serve-static.bats greps for the obsolete literal .js: application/javascript mapping. The implementation now correctly returns application/javascript; charset=utf-8, so the source-text assertion fails even though both executable behavior lanes pass.

Evidence

• test/serve-static-unit.bats passes the data-driven contentType() assertion.
• test/serve-static-e2e.bats passes the HTTP response header assertion.
• Full BATS_LIB_PATH=test/vendor bats -t test/*.bats is 110/111; this is the sole failure.

Acceptance

• Replace or remove the brittle source-text grep in favor of observable behavior.
• Keep unit and HTTP end-to-end coverage for the JavaScript content type.
• Full repository Bats is green.
• pnpm run preflight remains green.

Found during the five-slice documentation reconstruction drift check; not caused by #747.

[coderabbitai]

🔗 Related PRs

#731 - Publish extension generation provenance contract [merged]

---

📝 Issue Planner

_{Check the box below or use the @coderabbitai plan command to generate an implementation plan and prompts that you can use with your favorite coding assistant.}

• Create Plan

---

🧪 Issue enrichment is currently in open beta.

You can configure auto-planning by selecting labels in the issue_enrichment configuration.

To disable automatic issue enrichment, add the following to your .coderabbit.yaml:

issue_enrichment:
  auto_enrich:
    enabled: false

💬 Have feedback or questions? Drop into our discord!