fix: preserve user-specified Content-Type header in Response

[akapug]

Summary

Fixes #1817 - Response Content-Type header is overwritten by mapResponseBody()

Problem

When using elide serve with a fetch handler that returns a Response with a custom Content-Type header, the header was being unconditionally overwritten based on body type:

// User code
return new Response("<h1>Hello</h1>", { 
  headers: { "Content-Type": "text/html; charset=utf-8" }
});

// Actual response: Content-Type: text/plain (wrong!)

Note: This only affects the declarative fetch handler pattern (elide serve). The node:http imperative pattern (used by apps like compare-gpt-elide) is not affected.

Regression Source

Bug introduced in PR #1736 ("feat: new server engine"). The mapResponseBody() function was added with unconditional headers.set("Content-Type", ...) calls.

Solution

Modified mapResponseBody() to check if Content-Type is already set before applying defaults:

// Before (bug):
headers.set("Content-Type", "text/plain")

// After (fix):
if (!headers.has("Content-Type")) headers.set("Content-Type", "text/plain")

This preserves user-specified Content-Type headers while still providing sensible defaults when none is specified.

Testing

Added regression tests in FetchIntrinsicTest.kt:

• ✅ response should preserve user-specified Content-Type header
• ✅ response should use default Content-Type when not specified
• ✅ response should preserve custom headers

Manual testing:

• Tested with elide serve - custom Content-Type: text/html now preserved
• Custom headers like X-Custom were already working (only Content-Type was affected)

Changelog

• fix(graalvm): preserve user-specified Content-Type header in Response
• test(graalvm): add regression tests for Content-Type header preservation