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/chatmodelconfigs.sql

@@ -46,6 +46,21 @@ ORDER BY
     cmc.updated_at DESC,
     cmc.id DESC;
 
+-- name: GetEnabledChatModelConfigByID :one
+SELECT
+    cmc.*
+FROM
+    chat_model_configs cmc
+-- Providers can be disabled independently of their model configs.
+-- Check both to ensure the selected config is actually usable.
+JOIN
+    chat_providers cp ON cp.provider = cmc.provider
+WHERE
+    cmc.id = @id::uuid
+    AND cmc.deleted = FALSE
+    AND cmc.enabled = TRUE
+    AND cp.enabled = TRUE;
+
 -- name: InsertChatModelConfig :one
 INSERT INTO chat_model_configs (
     provider,