Function reserveFile

reserveFile(
originalFilename: string,
config: FileServingConfig,
): Promise<FileReservation>
Reserve a file location for streaming writes

Generates a validated ID and returns all paths needed to write the file. Does NOT create the file - caller should write to fullPath using streams or other means.
Parameters
- originalFilename: string
  Original filename to preserve
- config: FileServingConfig
  File serving configuration
Returns Promise<FileReservation>
FileReservation with id, storedName, and fullPath
Example
```
const reservation = await reserveFile('export.csv', {
  resourceStoreUri: 'file:///tmp/files',
  delimiter: '-'
});

const stream = createWriteStream(reservation.fullPath);
// ... write CSV incrementally ...
stream.end();

const uri = getFileUri(reservation.storedName, transport, uriConfig);
```
- Defined in src/file-serving/utils.ts:137