fix(extensions): prompt connection picker for extension commands when target is ambiguous

[ZhuchkaTriplesix]

Summary

In lib/core/extensions/extension_driver_session.dart, targetForExtension resolves which live session should execute a command triggered from Command Palette (Ctrl+P):

ExtensionCommandTarget targetForExtension(
  String extensionId, {
  int? preferredConnectionId,
}) {
  return resolveExtensionCommandTarget(
    extensionId: extensionId,
    liveConnectionIds: [
      for (final entry in _bridges.entries)
        if (entry.value.isStarted) entry.key,
    ],
    extensionIdFor: (id) => _manifests[id]?.id,
    preferredConnectionId: preferredConnectionId,
  );
}

If a user has multiple active connections of the same extension driver (e.g. "ClickHouse Dev" and "ClickHouse Prod") and triggers an extension command (e.g. "Optimize Partition" or "Inspect Storage") while focused on a neutral tab, resolveExtensionCommandTarget arbitrarily picks the first live connection ID.

This can result in executing maintenance or mutation commands on the wrong server (e.g. inadvertently running against production).

Scope

• When multiple live connections for the same extension exist and neither is active in the current workspace tab, do not silently pick the first connection ID.
• Prompt the user with a quick picker modal / bottom sheet to select the target connection before executing the command.
• Add test cases for multi-session command targeting.

Out of scope

• Built-in SQL workspace connection switching.

[ZhuchkaTriplesix]

Resolved by #1002 (merged into dev).