fix: improve SSH tunnel credential validation and error handling

[sentry]

This PR addresses an issue where warehouse connections configured with SSH tunneling could fail with a KeyError if required SSH credentials (specifically ssh_key) were missing from the stored configuration. Previously, this resulted in a generic HttpError: Failed to get schemas (500 Internal Server Error).

Changes Made:

1. ddpui/utils/helpers.py - map_airbyte_keys_to_postgres_keys:
   • Replaced hard bracket access (method["ssh_key"], method["tunnel_host"], etc.) with safe .get() calls.
   • Introduced explicit validation for the presence of all required SSH tunnel fields (tunnel_host, tunnel_port, tunnel_user, and ssh_key for key-based authentication).
   • If any required field is missing, a ValueError is now raised with a descriptive message.
2. ddpui/core/warehousefunctions.py - get_warehouse_data:
   • Modified the exception handling to specifically catch ValueError (raised by the helper function).
   • When a ValueError is caught, an HttpError(400) (Bad Request) is now returned to the user, providing a clear, actionable message about the misconfigured credentials, instead of a generic 500 error.

This ensures that misconfigured SSH tunnel connections provide a user-friendly error message, improving the debugging experience and preventing unexpected server errors.

Fixes DALGO-BACKEND-2BE