refactor: drop chat_model_configs provider column (#26877)

The provider type already lives authoritatively in ai_providers.type,
reachable on every active row through ai_provider_id, which the
chat_model_configs_ai_provider_required_when_active CHECK makes
mandatory. The stored provider string was a denormalized copy the system
kept in sync with a startup backfill and no longer needs.

Every surface now derives provider type from the linked ai_providers
row. Telemetry is the one exception: it keeps emitting provider, now
sourced from ai_providers.type via a JOIN, so the BigQuery column and the
Nexus dashboards that read it are unaffected. The experimental HTTP/SDK
response drops provider and makes ai_provider_id required, since those
endpoints return only active configs; consumers resolve provider type
from ai_provider_id and the AI providers listing.

This ships in a single release with no compatibility window: production
reads the table via SELECT *, so a pre-drop binary fails config reads the
moment the column is gone. Operators must scale to zero before upgrading,
and there is no rollback.

Closes CODAGT-599

codersdk/chats.go

@@ -1249,8 +1249,7 @@ type CreateUserChatProviderKeyRequest struct {
 // ChatModelConfig is an admin-managed model configuration.
 type ChatModelConfig struct {
 	ID                   uuid.UUID            `json:"id" format:"uuid"`
-	Provider             string               `json:"provider"`
-	AIProviderID         *uuid.UUID           `json:"ai_provider_id,omitempty" format:"uuid"`
+	AIProviderID         uuid.UUID            `json:"ai_provider_id" format:"uuid"`
 	Model                string               `json:"model"`
 	DisplayName          string               `json:"display_name"`
 	Enabled              bool                 `json:"enabled"`
@@ -1461,7 +1460,6 @@ func (c *ChatModelCallConfig) UnmarshalJSON(data []byte) error {
 
 // CreateChatModelConfigRequest creates a chat model config.
 type CreateChatModelConfigRequest struct {
-	Provider             string               `json:"provider,omitempty"`
 	AIProviderID         *uuid.UUID           `json:"ai_provider_id,omitempty" format:"uuid"`
 	Model                string               `json:"model"`
 	DisplayName          string               `json:"display_name,omitempty"`
@@ -1474,7 +1472,6 @@ type CreateChatModelConfigRequest struct {
 
 // UpdateChatModelConfigRequest updates a chat model config.
 type UpdateChatModelConfigRequest struct {
-	Provider             string               `json:"provider,omitempty"`
 	AIProviderID         *uuid.UUID           `json:"ai_provider_id,omitempty" format:"uuid"`
 	Model                string               `json:"model,omitempty"`
 	DisplayName          string               `json:"display_name,omitempty"`