Import-file data.id should be nullable

`import-file` is not working for new file updates since it requires id to get through schema validation https://github.com/directus/mcp/blob/472ce5b8dd3ddda45dba2a494cc024467a601ff4/src/types/files.ts#L5

Maybe it's just gpt4.1 that bad at MCP, but somehow it really struggles with this.

```
Result
Error processing tool: Received tool input did not match expected schema
The CMS is still reporting an input schema error, even with only the mandatory fields.
This strongly suggests that the CMS import function likely requires a specific field that hasn't been provided just yet—commonly, this could be a required folder ID or another identifier.
```