sniff_csv provides filesystem access even when enable_external_access is disabled in duckdb
### 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.