DuckDB allows access to files when enable_external_access is disabled

### Summary

Content in filesystem is accessible for reading using `sniff_csv`, even with `enable_external_access=false`.

### Details

During a pentest, a security researcher was able to access environment variable data and other system data by using the `sniff_csv` function, even though we set `enable_external_access` to false.

### PoC

```SQL
SET enable_external_access=false;
SET lock_configuration=true;

SELECT Columns FROM sniff_csv('/proc/self/environ');
```

### Impact

Provides an attacker with access to filesystem even when access is expected to be disabled and other similar functions do NOT provide access.

For example `select * from read_csv('/proc/self/environ')` fails with a permission error.

There seems to be two vectors to this vulnerability:

1. Access to files that should otherwise not be allowed. (We expect `Permission Error: Scanning CSV files is disabled through configuration` and not to provide any access to the file or even acknowledge that it exists).
2. The content from a (non-csv?) file can be read (e.g. `/etc/hosts`, `proc/self/environ`, etc) even though that doesn't seem to be the intent of the sniff_csv function (my understanding is it's intending to provide information about the shape of the data, but not provide the data itself).

### Workaround

It is possible to work around the issue by disabling the local file system using the `disabled_filesystems` setting:

```SQL
SET disabled_filesystems='LocalFileSystem';
SET lock_configuration=true;

SELECT Columns FROM sniff_csv('/proc/self/environ');
```

will result in `Permission Error: File system LocalFileSystem has been disabled by configuration`.

### Fix

A fix has been merged into the `main` branch (https://github.com/duckdb/duckdb/pull/13133), and will be released with the next DuckDB release.