Show a clear error when a connection's database driver isn't installed

[chrisclark]

Problem

A connection whose engine driver isn't installed (for example MySQL without mysqlclient) could be saved. After that, everything that touched it failed with a blank 500:

• the schema pane, which shows a "sad page" icon because Django's 500 page sends X-Frame-Options: DENY inside the iframe
• schema.json
• Test connection
• running queries

The cause: load_backend() raises ImproperlyConfigured, but as_django_connection() only caught DatabaseError. An unreachable database also 500'd the schema pane, because SchemaView didn't catch the OperationalError.

We found this on demo.sqlexplorer.io, where a visitor had added a MySQL connection.

Changes

• Model: as_django_connection() turns a missing driver into a DatabaseError that names the package to install. The existing DatabaseError handling then shows it for query runs and Test connection.
• Form: DatabaseConnectionForm refuses to save an engine whose driver can't be loaded, and labels those engines "(driver not installed)" in the dropdown. Test connection shows the specific message instead of "Invalid form data".
• Schema pane: SchemaView renders the existing schema_error.html with the actual error, instead of a 500.
• Schema JSON: schema_json_info() returns {} on failure. It used to return [], which made JsonResponse raise, so the existing InvalidExplorerConnectionException path also 500'd.

Testing

• New tests cover the model, form, schema view, schema JSON and validate view.
• Full suite: 284 tests pass under explorer.tests.settings and under settings_base (50 skipped). ruff==0.16.8 check explorer is clean.
• End to end, against a real missing mysqlclient with no mocks:
  • the schema pane returns 200 with SAMEORIGIN and shows the install hint
  • schema.json returns {}
  • Test connection returns the message
  • saving the form is refused
  • the dropdown flags the 6 engines whose drivers aren't installed

🤖 Generated with Claude Code