V2 Make FileId type opaque and use consistently throughout project (#4307)

# Description of Changes
The `FileId` type in V2 currently is just defined to be a string. This
makes it really easy to accidentally pass strings into things accepting
file IDs (such as file names). This PR makes the `FileId` type [an
opaque
type](https://www.geeksforgeeks.org/typescript/opaque-types-in-typescript/),
so it is compatible with things accepting strings (arguably not ideal
for this...) but strings are not compatible with it without explicit
conversion.

The PR also includes changes to use `FileId` consistently throughout the
project (everywhere I could find uses of `fileId: string`), so that we
have the maximum benefit from the type safety.

> [!note]
> I've marked quite a few things as `FIX ME` where we're passing names
in as IDs. If that is intended behaviour, I'm happy to remove the fix me
and insert a cast instead, but they probably need comments explaining
why we're using a file name as an ID.

frontend/src/hooks/tools/shared/useToolOperation.ts

@@ -8,6 +8,7 @@ import { useToolResources } from './useToolResources';
 import { extractErrorMessage } from '../../../utils/toolErrorHandler';
 import { createOperation } from '../../../utils/toolOperationTracker';
 import { ResponseHandler } from '../../../utils/toolResponseProcessor';
+import { FileId } from '../../../types/file';
 
 // Re-export for backwards compatibility
 export type { ProcessingProgress, ResponseHandler };
@@ -231,7 +232,7 @@ export const useToolOperation = <TParams>(
         actions.setDownloadInfo(downloadInfo.url, downloadInfo.filename);
 
         // Replace input files with processed files (consumeFiles handles pinning)
-        const inputFileIds = validFiles.map(file => findFileId(file)).filter(Boolean) as string[];
+        const inputFileIds = validFiles.map(file => findFileId(file)).filter(Boolean) as FileId[];
         await consumeFiles(inputFileIds, processedFiles);
 
         markOperationApplied(fileId, operationId);