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/middleware/auth.ts

@@ -11,6 +11,11 @@ export interface ProxyAuthContext {
   policy: DownstreamRoutingPolicy;
 }
 
+export interface ProxyResourceOwner {
+  ownerType: 'managed_key' | 'global_proxy_token';
+  ownerId: string;
+}
+
 const proxyAuthContextByRequest = new WeakMap<FastifyRequest, ProxyAuthContext>();
 
 function normalizeIp(rawIp: string | null | undefined): string {
@@ -107,3 +112,20 @@ export async function proxyAuthMiddleware(request: FastifyRequest, reply: Fastif
 export function getProxyAuthContext(request: FastifyRequest): ProxyAuthContext | null {
   return proxyAuthContextByRequest.get(request) || null;
 }
+
+export function getProxyResourceOwner(request: FastifyRequest): ProxyResourceOwner | null {
+  const auth = getProxyAuthContext(request);
+  if (!auth) return null;
+
+  if (auth.source === 'managed') {
+    return {
+      ownerType: 'managed_key',
+      ownerId: auth.keyId === null ? auth.token : String(auth.keyId),
+    };
+  }
+
+  return {
+    ownerType: 'global_proxy_token',
+    ownerId: 'global',
+  };
+}