feat: add internal subagent model override wiring (#24399)

> Mux working on behalf of Mike.

## Summary
- add an enabled chat model config lookup by ID for internal callers
- keep `spawn_agent` unchanged while threading an internal model
override through child subagent chat creation
- extend chatd coverage for inherited bindings, plan mode, and internal
override behavior

## Validation
- `go test ./coderd/x/chatd ./coderd/database/dbauthz`
- `make lint`

coderd/database/queries.sql.go

@@ -4274,6 +4274,45 @@ func (q *sqlQuerier) GetDefaultChatModelConfig(ctx context.Context) (ChatModelCo
 	return i, err
 }
 
+const getEnabledChatModelConfigByID = `-- name: GetEnabledChatModelConfigByID :one
+SELECT
+    cmc.id, cmc.provider, cmc.model, cmc.display_name, cmc.created_by, cmc.updated_by, cmc.enabled, cmc.is_default, cmc.deleted, cmc.deleted_at, cmc.created_at, cmc.updated_at, cmc.context_limit, cmc.compression_threshold, cmc.options
+FROM
+    chat_model_configs cmc
+JOIN
+    chat_providers cp ON cp.provider = cmc.provider
+WHERE
+    cmc.id = $1::uuid
+    AND cmc.deleted = FALSE
+    AND cmc.enabled = TRUE
+    AND cp.enabled = TRUE
+`
+
+// Providers can be disabled independently of their model configs.
+// Check both to ensure the selected config is actually usable.
+func (q *sqlQuerier) GetEnabledChatModelConfigByID(ctx context.Context, id uuid.UUID) (ChatModelConfig, error) {
+	row := q.db.QueryRowContext(ctx, getEnabledChatModelConfigByID, id)
+	var i ChatModelConfig
+	err := row.Scan(
+		&i.ID,
+		&i.Provider,
+		&i.Model,
+		&i.DisplayName,
+		&i.CreatedBy,
+		&i.UpdatedBy,
+		&i.Enabled,
+		&i.IsDefault,
+		&i.Deleted,
+		&i.DeletedAt,
+		&i.CreatedAt,
+		&i.UpdatedAt,
+		&i.ContextLimit,
+		&i.CompressionThreshold,
+		&i.Options,
+	)
+	return i, err
+}
+
 const getEnabledChatModelConfigs = `-- name: GetEnabledChatModelConfigs :many
 SELECT
     cmc.id, cmc.provider, cmc.model, cmc.display_name, cmc.created_by, cmc.updated_by, cmc.enabled, cmc.is_default, cmc.deleted, cmc.deleted_at, cmc.created_at, cmc.updated_at, cmc.context_limit, cmc.compression_threshold, cmc.options