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

coderd/database/querier.go

@@ -6,7 +6,6 @@ package database
 
 import (
 	"context"
-	"database/sql"
 	"encoding/json"
 	"time"
 
@@ -71,11 +70,6 @@ type sqlcQuerier interface {
 	// created_at ASC flows through to dbpurge's digest truncation; see
 	// buildDigestData in dbpurge.go for the tradeoff rationale.
 	AutoArchiveInactiveChats(ctx context.Context, arg AutoArchiveInactiveChatsParams) ([]AutoArchiveInactiveChatsRow, error)
-	// old_provider is matched as text; new_provider is also cast to ai_provider_type
-	// for the EXISTS check against ai_providers.type.
-	// ai_provider_id IS NOT NULL is defensive; the check constraint already
-	// enforces that non-deleted rows always have a provider ID.
-	BackfillChatModelConfigProvider(ctx context.Context, arg BackfillChatModelConfigProviderParams) (sql.Result, error)
 	BackoffChatDiffStatus(ctx context.Context, arg BackoffChatDiffStatusParams) error
 	// Deletes heartbeat rows for the supplied (chat_id, runner_id) pairs.
 	BatchDeleteChatHeartbeats(ctx context.Context, arg BatchDeleteChatHeartbeatsParams) (int64, error)
@@ -150,7 +144,6 @@ type sqlcQuerier interface {
 	DeleteChatDebugDataByChatID(ctx context.Context, arg DeleteChatDebugDataByChatIDParams) (int64, error)
 	DeleteChatModelConfigByID(ctx context.Context, id uuid.UUID) error
 	DeleteChatModelConfigsByAIProviderID(ctx context.Context, aiProviderID uuid.UUID) error
-	DeleteChatModelConfigsByProvider(ctx context.Context, provider string) error
 	DeleteChatQueuedMessage(ctx context.Context, arg DeleteChatQueuedMessageParams) error
 	// Deletes a queued message, scoped to the parent chat. Returns the
 	// number of affected rows so callers can detect missing rows without
@@ -449,6 +442,7 @@ type sqlcQuerier interface {
 	GetChatModelConfigByID(ctx context.Context, id uuid.UUID) (ChatModelConfig, error)
 	GetChatModelConfigs(ctx context.Context) ([]ChatModelConfig, error)
 	// Returns all model configurations for telemetry snapshot collection.
+	// deleted = false guarantees ai_provider_id is non-null, so INNER JOIN is safe.
 	GetChatModelConfigsForTelemetry(ctx context.Context) ([]GetChatModelConfigsForTelemetryRow, error)
 	// GetChatPersonalModelOverridesEnabled returns whether users may configure
 	// personal chat model overrides. It defaults to false when unset.
@@ -539,7 +533,7 @@ type sqlcQuerier interface {
 	// Providers can be disabled independently of their model configs.
 	// Check both to ensure the selected config is actually usable.
 	GetEnabledChatModelConfigByID(ctx context.Context, id uuid.UUID) (ChatModelConfig, error)
-	GetEnabledChatModelConfigs(ctx context.Context) ([]ChatModelConfig, error)
+	GetEnabledChatModelConfigs(ctx context.Context) ([]GetEnabledChatModelConfigsRow, error)
 	GetEnabledMCPServerConfigs(ctx context.Context) ([]MCPServerConfig, error)
 	// GetExternalAgentTokensByTemplateID returns the auth tokens for all
 	// non-deleted external agents on the latest build of every running workspace