fix(coderd): allow deleting chat providers used in historical chats (#24568)

Drop the `chat_model_configs.provider -> chat_providers.provider`
foreign key and soft-delete model configs when their provider is
removed. The provider row is now hard-deleted inside a transaction that
also tombstones its model configs and promotes a replacement default
when needed.

Historical chats and messages keep pointing at the soft-deleted model
config rows, which are hidden from live/admin queries but still resolve
for read. The runtime chat path already falls back to the default model
config when a soft-deleted config is looked up.

Replaces the lost FK validation in the create/update model-config
handlers with an explicit provider lookup that returns the existing
`Chat provider is not configured.` 400.

## UX

**Admin deleting a chat provider that has historical usage**

- Before: blocked with 400 `Provider models are still referenced by
existing chats.` Admins had no in-product way to remove a provider that
had ever been used.
- After: delete succeeds (204). Any model configs under that provider
are soft-deleted. If the removed provider owned the default model
config, one of the remaining live configs is auto-promoted to the new
default. The promotion is deterministic (`ensureDefaultChatModelConfig`
picks the first live config by `provider ASC, model ASC, updated_at
DESC, id DESC`); there is no picker, and no toast or response detail
names which config became the new default.

**End users with chats that used a deleted provider's model**

- Old chats still open and their history still renders unchanged.
- Sending a new turn in such a chat silently falls back to the current
default model. No banner or warning tells the user the original model is
gone.
- The model picker no longer lists the deleted model.
- If no default model config exists at all after the delete, sending a
new turn fails with `no default chat model config is available`.

**Admin creating or updating a model config against a provider that is
not configured**

- Same as before: 400 `Chat provider is not configured.` Only the
detection mechanism changed (explicit `FOR UPDATE` lookup inside the
transaction, which also serializes against a concurrent provider
delete).

**Admin updating a model config whose row disappears mid-transaction**

- Now returns the standard 404 `Resource not found or you do not have
access to this resource` instead of the previous 500 that leaked `sql:
no rows in result set` in the detail. Unrelated internal races (for
example a race on the promoted default candidate) are still reported as
500 so they are not misclassified as "your target is gone".

Closes CODAGT-23

coderd/database/dbauthz/dbauthz.go

@@ -1889,6 +1889,13 @@ func (q *querier) DeleteChatModelConfigByID(ctx context.Context, id uuid.UUID) e
 	return q.db.DeleteChatModelConfigByID(ctx, id)
 }
 
+func (q *querier) DeleteChatModelConfigsByProvider(ctx context.Context, provider string) error {
+	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceDeploymentConfig); err != nil {
+		return err
+	}
+	return q.db.DeleteChatModelConfigsByProvider(ctx, provider)
+}
+
 func (q *querier) DeleteChatProviderByID(ctx context.Context, id uuid.UUID) error {
 	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceDeploymentConfig); err != nil {
 		return err
@@ -2827,6 +2834,13 @@ func (q *querier) GetChatProviderByID(ctx context.Context, id uuid.UUID) (databa
 	return q.db.GetChatProviderByID(ctx, id)
 }
 
+func (q *querier) GetChatProviderByIDForUpdate(ctx context.Context, id uuid.UUID) (database.ChatProvider, error) {
+	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceDeploymentConfig); err != nil {
+		return database.ChatProvider{}, err
+	}
+	return q.db.GetChatProviderByIDForUpdate(ctx, id)
+}
+
 func (q *querier) GetChatProviderByProvider(ctx context.Context, provider string) (database.ChatProvider, error) {
 	if err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceDeploymentConfig); err != nil {
 		return database.ChatProvider{}, err
@@ -2834,6 +2848,13 @@ func (q *querier) GetChatProviderByProvider(ctx context.Context, provider string
 	return q.db.GetChatProviderByProvider(ctx, provider)
 }
 
+func (q *querier) GetChatProviderByProviderForUpdate(ctx context.Context, provider string) (database.ChatProvider, error) {
+	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceDeploymentConfig); err != nil {
+		return database.ChatProvider{}, err
+	}
+	return q.db.GetChatProviderByProviderForUpdate(ctx, provider)
+}
+
 func (q *querier) GetChatProviders(ctx context.Context) ([]database.ChatProvider, error) {
 	if err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceDeploymentConfig); err != nil {
 		return nil, err

coderd/database/dbauthz/dbauthz_test.go

@@ -449,6 +449,11 @@ func (s *MethodTestSuite) TestChats() {
 		dbm.EXPECT().DeleteChatModelConfigByID(gomock.Any(), id).Return(nil).AnyTimes()
 		check.Args(id).Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)
 	}))
+	s.Run("DeleteChatModelConfigsByProvider", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		providerName := "test-provider"
+		dbm.EXPECT().DeleteChatModelConfigsByProvider(gomock.Any(), providerName).Return(nil).AnyTimes()
+		check.Args(providerName).Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)
+	}))
 	s.Run("DeleteChatProviderByID", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
 		id := uuid.New()
 		dbm.EXPECT().DeleteChatProviderByID(gomock.Any(), id).Return(nil).AnyTimes()
@@ -803,12 +808,23 @@ func (s *MethodTestSuite) TestChats() {
 		dbm.EXPECT().GetChatProviderByID(gomock.Any(), provider.ID).Return(provider, nil).AnyTimes()
 		check.Args(provider.ID).Asserts(rbac.ResourceDeploymentConfig, policy.ActionRead).Returns(provider)
 	}))
+	s.Run("GetChatProviderByIDForUpdate", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
+		provider := testutil.Fake(s.T(), faker, database.ChatProvider{})
+		dbm.EXPECT().GetChatProviderByIDForUpdate(gomock.Any(), provider.ID).Return(provider, nil).AnyTimes()
+		check.Args(provider.ID).Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate).Returns(provider)
+	}))
 	s.Run("GetChatProviderByProvider", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
 		providerName := "test-provider"
 		provider := testutil.Fake(s.T(), faker, database.ChatProvider{Provider: providerName})
 		dbm.EXPECT().GetChatProviderByProvider(gomock.Any(), providerName).Return(provider, nil).AnyTimes()
 		check.Args(providerName).Asserts(rbac.ResourceDeploymentConfig, policy.ActionRead).Returns(provider)
 	}))
+	s.Run("GetChatProviderByProviderForUpdate", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
+		providerName := "test-provider"
+		provider := testutil.Fake(s.T(), faker, database.ChatProvider{Provider: providerName})
+		dbm.EXPECT().GetChatProviderByProviderForUpdate(gomock.Any(), providerName).Return(provider, nil).AnyTimes()
+		check.Args(providerName).Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate).Returns(provider)
+	}))
 	s.Run("GetChatProviders", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
 		providerA := testutil.Fake(s.T(), faker, database.ChatProvider{})
 		providerB := testutil.Fake(s.T(), faker, database.ChatProvider{})