fix(connection): bug in changing sql flavor

[aarthy-dk]

• Consolidated the connection form and widget keys to always use the same connection ID, since there is only one connection. Otherwise the form key was changing between "new" and "1" when the SQL flavor was changed, and the form was not keeping the correct state.
• Updated the Test Connection button to use the data on the next rerun. Otherwise, if the button was clicked immediately after changing a field (without unfocusing the input), the latest change was not used for validating the connection.
• Disabled the "connection name" field for simplicity since there is only one connection.

[github-actions]

Coverage Report •

File	Stmts	Miss	Cover	Missing
testgen/ui/views/connections
page.py	169	169	0%	1–331
TOTAL	8174	6536	17%

Tests	Skipped	Failures	Errors	Time
28	0 💤	0 ❌	0 🔥	3.576s ⏱️