fix(providers/openai): route gpt-5.6 family through Responses API

gpt-5.6-sol, gpt-5.6-terra, and gpt-5.6-luna are reasoning models and
must use the Responses API. Without allowlist entries they fell through
to the Chat Completions path, which breaks reasoning continuity for
agents.

Author: ibetitsmike

providers/openai/responses_options.go

@@ -235,6 +235,9 @@ var responsesReasoningModelIDs = []string{
 	"gpt-5.4-codex",
 	"gpt-5.5",
 	"gpt-5.5-pro",
+	"gpt-5.6-sol",
+	"gpt-5.6-terra",
+	"gpt-5.6-luna",
 	"gpt-oss-120b",
 }
 

providers/openai/responses_params_test.go

@@ -240,6 +240,15 @@ func TestValidatePreviousResponseIDPrompt(t *testing.T) {
 	}
 }
 
+func TestResponsesModelAllowlist_GPT56Family(t *testing.T) {
+	t.Parallel()
+
+	for _, modelID := range []string{"gpt-5.6-sol", "gpt-5.6-terra", "gpt-5.6-luna"} {
+		require.True(t, IsResponsesModel(modelID), modelID)
+		require.True(t, IsResponsesReasoningModel(modelID), modelID)
+	}
+}
+
 func TestResponsesProviderMetadata_Helper(t *testing.T) {
 	t.Parallel()