Add Infermatic provider test case

Adds test coverage for the Infermatic AI provider in
TestCreateClient to verify the provider exists and
creates a valid client.

Part of #1033: Add Infermatic AI provider support

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/plugins/ai/openai_compatible/providers_config_test.go

@@ -30,6 +30,11 @@ func TestCreateClient(t *testing.T) {
 			provider: "Abacus",
 			exists:   true,
 		},
+		{
+			name:     "Existing provider - Infermatic",
+			provider: "Infermatic",
+			exists:   true,
+		},
 		{
 			name:     "Existing provider - MiniMax",
 			provider: "MiniMax",