feat: track step runtime_ms on chat messages (#23219)

## Summary

Adds a `runtime_ms` column to `chat_messages` that records the
wall-clock duration (in milliseconds) of each LLM step. This covers LLM
streaming, tool execution, and retries — the full time the agent is
"alive" for a step.

This is the foundation for billing by agent alive time. The column
follows the same pattern as `total_cost_micros`: stored per assistant
message, aggregatable with `SUM()` over time periods by user.

## Changes

- **Migration**: adds nullable `runtime_ms bigint` to `chat_messages`.
- **chatloop**: adds `Runtime time.Duration` field to `PersistedStep`,
measures `time.Since(stepStart)` at the beginning of each step (covering
stream + tool execution + retries).
- **chatd**: passes `step.Runtime.Milliseconds()` to the assistant
message `InsertChatMessage` call; all other message types (system, user,
tool) get `NULL`.
- **Tests**: adds `runtime > 0` assertion in chatloop tests.

## Billing query pattern

Once ready, aggregation mirrors the existing cost queries:

```sql
SELECT COALESCE(SUM(cm.runtime_ms), 0)::bigint AS total_runtime_ms
FROM chat_messages cm
JOIN chats c ON c.id = cm.chat_id
WHERE c.owner_id = @user_id
  AND cm.created_at >= @start_time
  AND cm.created_at < @end_time
  AND cm.runtime_ms IS NOT NULL;
```

coderd/database/dump.sql

@@ -1289,7 +1289,8 @@ CREATE TABLE chat_messages (
     compressed boolean DEFAULT false NOT NULL,
     created_by uuid,
     content_version smallint NOT NULL,
-    total_cost_micros bigint
+    total_cost_micros bigint,
+    runtime_ms bigint
 );
 
 CREATE SEQUENCE chat_messages_id_seq