fix(files): only show loading skeleton while fetching, relax mongo auth validation

- files.tsx: gate loading skeleton on isLoading so navigating to a missing
  file ID doesn't permanently show the skeleton after load completes
- database-tools.ts: remove refine() pairing username+password on mongo
  connection schema so either field can be provided independently

Author: waleedlatif1

apps/sim/app/workspace/[workspaceId]/files/files.tsx

@@ -1033,7 +1033,7 @@ export function Files() {
     return tags
   }, [typeFilter, sizeFilter, uploadedByFilter, members])
 
-  if (fileIdFromRoute && !selectedFile) {
+  if (fileIdFromRoute && !selectedFile && isLoading) {
     return (
       <div className='flex h-full flex-1 flex-col overflow-hidden bg-[var(--bg)]'>
         <ResourceHeader icon={FilesIcon} breadcrumbs={loadingBreadcrumbs} />

apps/sim/lib/api/contracts/database-tools.ts

@@ -120,20 +120,15 @@ export const rdsIntrospectBodySchema = rdsConnectionBodySchema.extend({
   engine: z.enum(['aurora-postgresql', 'aurora-mysql']).optional(),
 })
 
-const mongoConnectionBodySchema = z
-  .object({
-    host: z.string().min(1, 'Host is required'),
-    port: z.coerce.number().int().positive('Port must be a positive integer'),
-    database: z.string().min(1, 'Database name is required'),
-    username: z.string().min(1, 'Username is required').optional(),
-    password: z.string().min(1, 'Password is required').optional(),
-    authSource: z.string().optional(),
-    ssl: sslModeSchema,
-  })
-  .refine((data) => Boolean(data.username) === Boolean(data.password), {
-    message: 'Username and password must be provided together',
-    path: ['password'],
-  })
+const mongoConnectionBodySchema = z.object({
+  host: z.string().min(1, 'Host is required'),
+  port: z.coerce.number().int().positive('Port must be a positive integer'),
+  database: z.string().min(1, 'Database name is required'),
+  username: z.string().min(1, 'Username is required').optional(),
+  password: z.string().min(1, 'Password is required').optional(),
+  authSource: z.string().optional(),
+  ssl: sslModeSchema,
+})
 
 const mongoJsonStringOrObjectSchema = (message: string) =>
   z