Pre-connect shell script not working with AWS RDS through Proxy RDS

## What did you do?

- I setup an AWS RDS database with an RDS Proxy
- I setup an SSH tunnel to reach the RDS Proxy to access the database
- I can connect with the `psql` CLI
- I created in Postico 2.1 a server configuration to reach my AWS RDS database through an RDS Proxy through an SSH tunnel with a pre-connect shell script
- **Unfortunately, the connection fails**
- I removed the RDS proxy,
- I can connect with the `psql` CLI
- I can connect with Postico

## What did you expect to happen?

- I expected to be able to connect through Postico onto the Proxy RDS

## What actually happened?

- I can see a loop with the following labels in the top bar during the connection :
- Negociating SSL
- Establishing SSH tunnel
- Executing password shell command
- Finally, I get this error message : `The server rejected the password provided by the pre-connect shell script.`
- However, the same script works for the same database if I remove the proxy
- I can connect to the database using `psql` CLI with the `sslmode=require` parameter

## What software versions are you using?

- Postico version: Version 2.1 (9682)
- macOS version: 14.4.1 (23E224)
- PostgreSQL version: `PostgreSQL 14.9 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 7.3.1 20180712 (Red Hat 7.3.1-12), 64-bit`

Postico server JSON configuration:
```json
{
"connection" : {
"postgres" : {
"database" : "bopgdb",
"host" : "my-rds-proxy.proxy-id.eu-west-1.rds.amazonaws.com",
"port" : 5432,
"preconnectScript" : {
"enabled" : true,
"source" : "export PGPASSWORD=\"$(aws rds generate-db-auth-token --hostname my-rds-proxy.proxy-id.eu-west-1.rds.amazonaws.com --port 5432 --region eu-west-1 --username myuser --profile myprofile)\"\n\necho $PGPASSWORD"
},
"user" : "myuser"
},
"ssh" : {
"enabled" : true,
"host" : "1.2.3.4",
"port" : 22,
"privateKey" : {
"data" : "xxx",
"type" : "bookmark"
},
"user" : "my-ec2-bastion-user"
}
},
"displayName" : "RDS through Proxy RDS",
"startupQuery" : {
"enabled" : false
},
"type" : "server",
"uuid" : "1326EB50-2A12-4D6F-801A-6DB21ED7A4A6",
"version" : 2
}
```

Start by reproducing the Postico 2.1 server configuration with the SSH tunnel, RDS Proxy endpoint, and pre-connect shell script, then compare it with the working psql command using sslmode=require. Trace the pre-connect password-shell and SSH-tunnel stages shown in the connection loop. Done means the same generated authentication token is accepted through the RDS Proxy.

Written by the indexing model from the issue text.