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/queries/chats.sql

@@ -2220,7 +2220,7 @@ WHERE
 SELECT
     cmc.id AS model_config_id,
     cmc.display_name,
-    cmc.provider,
+    COALESCE(ap.type::text, '')::text AS provider,
     cmc.model,
     COALESCE(SUM(cm.total_cost_micros), 0)::bigint AS total_cost_micros,
     COUNT(*) FILTER (
@@ -2241,13 +2241,15 @@ JOIN
     chats c ON c.id = cm.chat_id
 JOIN
     chat_model_configs cmc ON cmc.id = cm.model_config_id
+LEFT JOIN
+    ai_providers ap ON ap.id = cmc.ai_provider_id
 WHERE
     c.owner_id = @owner_id::uuid
     AND cm.role = 'assistant'
     AND cm.created_at >= @start_date::timestamptz
     AND cm.created_at < @end_date::timestamptz
 GROUP BY
-    cmc.id, cmc.display_name, cmc.provider, cmc.model
+    cmc.id, cmc.display_name, ap.type, cmc.model
 ORDER BY
     total_cost_micros DESC;
 
@@ -2580,9 +2582,11 @@ GROUP BY cm.chat_id;
 
 -- name: GetChatModelConfigsForTelemetry :many
 -- Returns all model configurations for telemetry snapshot collection.
-SELECT id, provider, model, context_limit, enabled, is_default
-FROM chat_model_configs
-WHERE deleted = false;
+-- deleted = false guarantees ai_provider_id is non-null, so INNER JOIN is safe.
+SELECT cmc.id, ap.type::text AS provider, cmc.model, cmc.context_limit, cmc.enabled, cmc.is_default
+FROM chat_model_configs cmc
+JOIN ai_providers ap ON ap.id = cmc.ai_provider_id
+WHERE cmc.deleted = false;
 -- name: GetActiveChatsByAgentID :many
 SELECT *
 FROM chats_expanded