fix(aws-lambda): add preview command

[taylorfsteele]

🔗 Linked issue

#4051

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Closes #4051

This PR adds a preview command for the aws-lambda preset.

In order to get the preview command working correctly, two additional changes were added:

• The aws-lambda and streaming presets now export nitroApp.fetch.
  • I plucked this concept from presets/standard/runtime/server.ts after running into issues where the preview command was unable to load correctly. I'm very open to excluding this from the PR and moving this to a more Vite or App level concern, as I'm not entirely unsure about the ramifications of this change. I'm not exactly sure how to do that, but let me know what you think.
• Added a shim that adds awslambda globals to the preview command.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📝 Walkthrough

Walkthrough

Adds AWS Lambda preset preview command support with streaming capabilities. Introduces default exports for fetch functionality in runtime presets and creates a preview shim utility for streaming mode compatibility.

Changes

Cohort / File(s)	Summary
Preset Configuration src/presets/aws-lambda/preset.ts	Adds preview command with streaming support, conditional shim import in preview input, and a compiled lifecycle hook that generates the AWS Lambda preview shim file to server output directory when streaming is enabled.
Runtime Exports src/presets/aws-lambda/runtime/aws-lambda.ts, src/presets/aws-lambda/runtime/aws-lambda-streaming.ts	Adds default exports exposing { fetch: nitroApp.fetch } to both standard and streaming runtime presets, enabling module-level import as default module with fetch method.
Utility Constants src/presets/aws-lambda/utils.ts	Introduces new awsLambdaPreviewShim string constant defining a minimal globalThis.awslambda compatibility shim with streamifyResponse method and HttpResponseStream.from utility for preview environments.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

• feat: nitro preview #4024: Adds AWS Lambda preview command infrastructure and streaming preview shim generation logic aligned with preset configuration changes.
• feat(vite): use srvx loader for preview #3999: Adds default fetch exports to runtime presets and defines AWS Lambda preview shim utility for streaming compatibility.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The PR title 'fix(aws-lambda): add preview command' follows conventional commits format with a fix type and clear scope.
Description check	✅ Passed	The PR description clearly explains the changes, links to issue #4051, describes the rationale for exports, and mentions the added shim.
Linked Issues check	✅ Passed	The PR fully addresses issue #4051 by adding a preview command for the aws-lambda preset, enabling TanStack Start prerendering compatibility.
Out of Scope Changes check	✅ Passed	All changes are directly related to adding preview command support for the aws-lambda preset, with no extraneous modifications detected.

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✨ Finishing Touches

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• Create PR with unit tests
• Post copyable unit tests in a comment

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commit: 66dd034