fix: use getAllInterfaceFields for http handler return type

[cs01]

Summary

• Fixes direct .fields access on interface in HTTP server handler return type detection
• llvm-generator.ts:2617 was using retIface.fields which only returns OWN fields, missing inherited fields from extends
• Now uses getAllInterfaceFields(retIface) to correctly detect headers and bodyLen fields on interfaces that inherit from a parent
• Without this fix, if an HTTP handler returns an interface that inherits headers/bodyLen from a parent interface, the server wouldn't generate the correct response handling code

Test plan

• All 434 tests pass
• Self-hosting verification passes (verify:quick)

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