fix: resolve issue with processing content-length incorrectly (#520)

## Description

This PR fixes issue with `content-length` header.

## Type of change

-   [x] Bug fix
-   [ ] New feature
-   [ ] Protocol integration
-   [ ] Documentation update
-   [ ] Other (please describe):

## Screenshots

None.

## Testing

`pnpm t`

## Related Issues

Relates to: #451

## Checklist

-   [x] My code follows the project's style guidelines
-   [x] I have added tests that prove my fix/feature works
-   [x] All tests pass locally and in CI
-   [ ] I have updated documentation as needed
-   [x] CI/CD checks pass
-   [ ] I have included screenshots for protocol screens (if applicable)
- [ ] For security-related features, I have included links to related
information

<!-- ELLIPSIS_HIDDEN -->

----

> [!IMPORTANT]
> Fixes handling of `Content-Length` header in `GET` function, ensuring
correct behavior when present or absent, with tests added.
> 
>   - **Behavior**:
> - Fixes handling of `Content-Length` header in `GET` function in
`route.ts`.
>     - Preserves `Content-Length` if present; omits if absent.
>     - Returns 400 error if required headers are missing.
>   - **Tests**:
> - Adds tests in `endpoint.test.ts` for responses with and without
`Content-Length`.
>     - Verifies correct status and headers in both scenarios.
>   - **Misc**:
> - Renames test suite from `metadata/[network] endpoint` to `Metadata
Proxy Route`.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis"
src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=solana-foundation%2Fexplorer&utm_source=github&utm_medium=referral)<sup>
for 6252226. It will automatically
update as commits are pushed.</sup>

<!-- ELLIPSIS_HIDDEN -->

Author: rogaldh

app/api/metadata/proxy/__tests__/endpoint.test.ts

@@ -49,7 +49,7 @@ function requestFactory(uri?: string) {
     return { nextParams, request };
 }
 
-describe('metadata/[network] endpoint', () => {
+describe('Metadata Proxy Route', () => {
     const validUrl = encodeURIComponent('http://external.resource/file.json');
     const unsupportedUri = encodeURIComponent('ftp://unsupported.resource/file.json');
 
@@ -104,3 +104,77 @@ describe('metadata/[network] endpoint', () => {
         expect((await GET(request.request, request.nextParams)).status).toBe(200);
     });
 });
+
+describe('Metadata Proxy Route :: resource fetching', () => {
+    const testUri = 'http://google.com/metadata.json';
+    const testData = { description: "Test Description", name: "Test NFT" };
+
+    beforeEach(() => {
+        process.env.NEXT_PUBLIC_METADATA_ENABLED = 'true';
+    });
+
+    it('should handle response without Content-Length header', async () => {
+        const sourceHeaders = new Headers({
+            'Cache-Control': 'max-age=3600',
+            'Content-Type': 'application/json',
+            'ETag': 'test-etag',
+        });
+
+        // @ts-expect-error lookup does not have mocked fn
+        dns.lookup.mockResolvedValueOnce([{ address: '8.8.8.8' }]);
+
+        // Mock fetch to return a response without Content-Length
+        // @ts-expect-error fetch does not have mocked fn
+        fetch.mockResolvedValueOnce({
+            arrayBuffer: async () => new ArrayBuffer(8),
+            headers: sourceHeaders,
+            json: async () => testData
+        });
+
+        const request = new Request(`http://localhost:3000/api/metadata/proxy?uri=${encodeURIComponent(testUri)}`);
+        const response = await GET(request, { params: {} });
+
+        // Verify response
+        expect(response.status).toBe(200);
+        expect(response.headers.get('content-type')).toBe('application/json');
+        expect(response.headers.get('cache-control')).toBe('max-age=3600');
+        expect(response.headers.get('etag')).toBe('test-etag');
+
+        // Next.js should calculate and set Content-Length automatically in theory
+        const contentLength = response.headers.get('content-length');
+        expect(contentLength).toBeNull();
+    });
+
+    it('should preserve original Content-Length header when present', async () => {
+        const originalContentLength = '89'; // Length of testData JSON, but different from real one
+        const sourceHeaders = new Headers({
+            'Cache-Control': 'max-age=3600',
+            'Content-Length': originalContentLength,
+            'Content-Type': 'application/json',
+            'ETag': 'test-etag',
+        });
+
+        // @ts-expect-error lookup does not have mocked fn
+        dns.lookup.mockResolvedValueOnce([{ address: '8.8.8.8' }]);
+
+        // Mock fetch to return a response with Content-Length
+        // @ts-expect-error fetch does not have mocked fn
+        fetch.mockResolvedValueOnce({
+            arrayBuffer: async () => new ArrayBuffer(8),
+            headers: sourceHeaders,
+            json: async () => testData
+        });
+
+        const request = new Request(`http://localhost:3000/api/metadata/proxy?uri=${encodeURIComponent(testUri)}`);
+        const response = await GET(request, { params: {} });
+
+        // Verify response
+        expect(response.status).toBe(200);
+        expect(response.headers.get('content-type')).toBe('application/json');
+        expect(response.headers.get('cache-control')).toBe('max-age=3600');
+        expect(response.headers.get('etag')).toBe('test-etag');
+
+        // Content-Length should match original
+        expect(response.headers.get('content-length')).toBe(originalContentLength);
+    });
+});

app/api/metadata/proxy/route.ts

@@ -89,13 +89,24 @@ export async function GET(
     }
 
     // preserve original cache-control headers
-    const responseHeaders = {
+    const contentLength = resourceHeaders.get('content-length');
+    const responseHeaders: Record<string, string> = {
         'Cache-Control': resourceHeaders.get('cache-control') ?? 'no-cache',
-        'Content-Length': resourceHeaders.get('content-length') as string,
-        'Content-Type': resourceHeaders.get('content-type') ?? 'application/json, charset=utf-8',
+        'Content-Type': resourceHeaders.get('content-type') ?? 'application/json; charset=utf-8',
         Etag: resourceHeaders.get('etag') ?? 'no-etag',
     };
 
+    // Only set Content-Length if it exists in the original response
+    if (contentLength) {
+        responseHeaders['Content-Length'] = contentLength;
+    }
+
+    // Validate that all required headers are present
+    const hasMissingHeaders = Object.values(responseHeaders).some(value => value == null);
+    if (hasMissingHeaders) {
+        return respondWithError(400);
+    }
+
     if (data instanceof ArrayBuffer) {
         return new NextResponse(data, {
             headers: responseHeaders,