fix: preserve AI provider preset types (#25925)

> Mux created this PR on behalf of Mike.

AI provider creation previously collapsed OpenAI-compatible presets like
Google and generic OpenAI-compatible providers to `openai`, which lost
the backend provider discriminator.

Preserve selected provider types in the create payload, keep explicit
stored types authoritative when reconstructing edit form values, and add
frontend plus backend regressions for the supported preset types.

coderd/ai_providers_test.go

@@ -44,6 +44,41 @@ func TestAIProvidersCRUD(t *testing.T) {
 		require.Empty(t, got)
 	})
 
+	t.Run("CreatePreservesPresetProviderTypes", func(t *testing.T) {
+		t.Parallel()
+		client := coderdtest.New(t, nil)
+		_ = coderdtest.CreateFirstUser(t, client)
+		ctx := testutil.Context(t, testutil.WaitLong)
+
+		tests := []struct {
+			providerType codersdk.AIProviderType
+			baseURL      string
+		}{
+			{providerType: codersdk.AIProviderTypeAzure, baseURL: "https://example.openai.azure.com/openai/v1"},
+			{providerType: codersdk.AIProviderTypeGoogle, baseURL: "https://generativelanguage.googleapis.com/v1beta/openai/"},
+			{providerType: codersdk.AIProviderTypeOpenAICompat, baseURL: "https://compat.example.com/v1"},
+			{providerType: codersdk.AIProviderTypeOpenrouter, baseURL: "https://openrouter.ai/api/v1"},
+			{providerType: codersdk.AIProviderTypeVercel, baseURL: "https://ai-gateway.vercel.sh/v1"},
+		}
+		for _, tt := range tests {
+			t.Run(string(tt.providerType), func(t *testing.T) {
+				created, err := client.CreateAIProvider(ctx, codersdk.CreateAIProviderRequest{
+					Type:    tt.providerType,
+					Name:    "type-preserve-" + string(tt.providerType),
+					Enabled: true,
+					BaseURL: tt.baseURL,
+					APIKeys: []string{"sk-test"},
+				})
+				require.NoError(t, err, tt.providerType)
+				require.Equal(t, tt.providerType, created.Type)
+
+				got, err := client.AIProvider(ctx, created.ID.String())
+				require.NoError(t, err, tt.providerType)
+				require.Equal(t, tt.providerType, got.Type)
+			})
+		}
+	})
+
 	t.Run("CreateGetUpdateDelete", func(t *testing.T) {
 		t.Parallel()
 		client := coderdtest.New(t, nil)