fix: chatd refactor (#26270)

Implements the chatd stabilization RFC.

Combines:
- https://github.com/coder/coder/pull/25908
- https://github.com/coder/coder/pull/25923
- https://github.com/coder/coder/pull/26109
- https://github.com/coder/coder/pull/26110
- https://github.com/coder/coder/pull/26111
- https://github.com/coder/coder/pull/26112

coderd/database/migrations/testdata/fixtures/000519_chatd_core_state_machine.up.sql

@@ -0,0 +1,17 @@
+-- Fixture coverage for the chat_heartbeats table introduced in
+-- migration 000500. The earlier chat fixtures already insert at least
+-- one row into chats; we attach a heartbeat for the first such chat so
+-- migration tests see a non-empty chat_heartbeats table without
+-- hard-coding a specific chat ID.
+INSERT INTO chat_heartbeats (
+    chat_id,
+    runner_id,
+    heartbeat_at
+)
+SELECT
+    chats.id,
+    '00000000-0000-0000-0000-0000000fea51'::uuid,
+    '2024-01-01 00:00:00+00'
+FROM chats
+ORDER BY created_at, id
+LIMIT 1;