fix(connections): editing an extension connection whose driver is missing does nothing, or opens another driver's form #1314

Problem

promptEditConnection (lib/features/connections/connection_creation_flow.dart):

• When the extension's manifest is not found (extension removed or disabled), it returns null silently: Edit appears to do nothing.
• _driverForConnection falls back to the first driver of the manifest when no driverId matches, so the form of a different driver opens for the connection and saving can change its meaning.
• An unknown connection type also returns null without a message.

Scope

• Tell the user why: The extension "X" that provides this connection is not installed or is disabled (with an action to open the extension manager).
• No fallback to the first driver; report the missing driver.

Acceptance

• Edit on a connection of an uninstalled extension shows a message.
• A manifest without the connection's driver does not open another driver's form.