Permissions error in file-transfer service is treated as transient when it will never resolve itself

[jarhodes314]

Describe the bug
When triggering a configuration snapshot where tedge does not have permissions to write to/create /var/tedge/file-transfer/<device_id>/, the file-transfer service generates a 500 internal server error. This appears to be treated as a transient error by the uploader code, so it attempts to retry the upload even though it is essentially guaranteed to fail without some human intervention.

To Reproduce

1. Delete /var/tedge/file-transfer/<device_id>/
2. sudo chown root "/var/tedge/file-transfer/ && sudo chmod 755 /var/tedge/file-transfer
3. Trigger a configuration-snapshot operation in Cumulocity

The operation gets stuck in the executing state. You can confirm there is a permissions error through observing the following:

$ systemctl status tedge-agent
Jan 31 17:45:21 james-Latitude-5511 tedge-agent[687003]:     Permission denied (os error 13)
Jan 31 17:45:21 james-Latitude-5511 tedge-agent[687003]: 2025-01-31T17:45:21.24358042Z  WARN upload::upload: Temporary failure: HTTP status server error (500 Internal Server Error) for url (http://127.0.0.1:8000/tedge/file-transfer/jrh-test.5/config_snapshot/tedge-configuration-plugin-c8y-mapper-141376). Retrying in 22s
Jan 31 17:45:43 james-Latitude-5511 tedge-agent[687003]: 2025-01-31T17:45:43.741483458Z ERROR tedge_agent::http_server::error: Request to upload to "jrh-test.5/config_snapshot/tedge-configuration-plugin-c8y-mapper-141376" failed: Directory Error. Check permissions for /var/tedge/file-transfer/jrh-test.5/config_snapshot.
Jan 31 17:45:43 james-Latitude-5511 tedge-agent[687003]: Caused by:
Jan 31 17:45:43 james-Latitude-5511 tedge-agent[687003]:     Permission denied (os error 13)
Jan 31 17:45:43 james-Latitude-5511 tedge-agent[687003]: 2025-01-31T17:45:43.741968038Z  WARN upload::upload: Temporary failure: HTTP status server error (500 Internal Server Error) for url (http://127.0.0.1:8000/tedge/file-transfer/jrh-test.5/config_snapshot/tedge-configuration-plugin-c8y-mapper-141376). Retrying in 36s
Jan 31 17:46:20 james-Latitude-5511 tedge-agent[687003]: 2025-01-31T17:46:20.488489158Z ERROR tedge_agent::http_server::error: Request to upload to "jrh-test.5/config_snapshot/tedge-configuration-plugin-c8y-mapper-141376" failed: Directory Error. Check permissions for /var/tedge/file-transfer/jrh-test.5/config_snapshot.
Jan 31 17:46:20 james-Latitude-5511 tedge-agent[687003]: Caused by:
Jan 31 17:46:20 james-Latitude-5511 tedge-agent[687003]:     Permission denied (os error 13)
Jan 31 17:46:20 james-Latitude-5511 tedge-agent[687003]: 2025-01-31T17:46:20.489027966Z  WARN upload::upload: Temporary failure: HTTP status server error (500 Internal Server Error) for url (http://127.0.0.1:8000/tedge/file-transfer/jrh-test.5/config_snapshot/tedge-configuration-plugin-c8y-mapper-141376). Retrying in 45s

Expected behavior
If the uploader cannot upload to the file transfer service because of a misconfiguration such as this it should abort the request rather than retrying it. I suspect we just want to have different transient-error detection behaviour depending on whether we are connecting to an internal API (where it is probably reasonable to assume a 500 is a permanent error) or an external one (including the Cumulocity proxy, as that will echo the status code from Cumulocity).