feat: implement proxy input file resolution and storage

- Added `inputFiles.ts` for handling input file resolution, including validation and normalization of file types.
- Introduced `proxyFileStore.ts` for managing file storage, retrieval, and deletion with owner-based isolation.
- Created tests for proxy file storage functionality in `proxyFileStore.test.ts`.
- Developed `proxyInputFileResolver.ts` to resolve input files in OpenAI and response formats.
- Added utility functions for handling reasoning signatures in `reasoningTransport.ts` and corresponding tests.
- Enhanced shared input file transformers in `inputFile.ts` for better file handling and type inference.

src/server/services/proxyInputFileResolver.test.ts

@@ -0,0 +1,81 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+const getProxyFileByPublicIdForOwnerMock = vi.fn();
+
+vi.mock('./proxyFileStore.js', () => ({
+  getProxyFileByPublicIdForOwner: (...args: unknown[]) => getProxyFileByPublicIdForOwnerMock(...args),
+  LOCAL_PROXY_FILE_ID_PREFIX: 'file-metapi-',
+}));
+
+describe('proxyInputFileResolver', () => {
+  beforeEach(() => {
+    getProxyFileByPublicIdForOwnerMock.mockReset();
+  });
+
+  it('preserves non-local file ids without resolving them from the local store', async () => {
+    const { resolveResponsesBodyInputFiles } = await import('./proxyInputFileResolver.js');
+    const body = {
+      model: 'gpt-5',
+      input: [
+        {
+          type: 'message',
+          role: 'user',
+          content: [
+            {
+              type: 'input_file',
+              file_id: 'file_external_123',
+            },
+          ],
+        },
+      ],
+    };
+
+    await expect(resolveResponsesBodyInputFiles(
+      body,
+      { ownerType: 'global_proxy_token', ownerId: 'global' },
+    )).resolves.toEqual(body);
+    expect(getProxyFileByPublicIdForOwnerMock).not.toHaveBeenCalled();
+  });
+
+  it('resolves object-form responses input payloads with local file ids', async () => {
+    getProxyFileByPublicIdForOwnerMock.mockResolvedValue({
+      publicId: 'file-metapi-123',
+      filename: 'brief.pdf',
+      mimeType: 'application/pdf',
+      contentBase64: Buffer.from('%PDF-local').toString('base64'),
+    });
+
+    const { resolveResponsesBodyInputFiles } = await import('./proxyInputFileResolver.js');
+    await expect(resolveResponsesBodyInputFiles(
+      {
+        model: 'gpt-5',
+        input: {
+          type: 'message',
+          role: 'user',
+          content: [
+            {
+              type: 'input_file',
+              file_id: 'file-metapi-123',
+            },
+          ],
+        },
+      },
+      { ownerType: 'managed_key', ownerId: '9' },
+    )).resolves.toEqual({
+      model: 'gpt-5',
+      input: {
+        type: 'message',
+        role: 'user',
+        content: [
+          {
+            type: 'input_file',
+            file_id: 'file-metapi-123',
+            filename: 'brief.pdf',
+            file_data: Buffer.from('%PDF-local').toString('base64'),
+            mime_type: 'application/pdf',
+          },
+        ],
+      },
+    });
+  });
+});