feat(coderd): return log snapshot for paused tasks (#21771)

Previously the task logs endpoint only worked when the workspace was
running, leaving users unable to view task history after pausing.

This change adds snapshot retrieval with state-based branching: active
tasks fetch live logs from AgentAPI, paused/initializing/pending tasks
return stored snapshots (providing continuity during pause/resume), and
error/unknown states return HTTP 409 Conflict.

The response includes snapshot metadata (snapshot, snapshot_at) to
indicate whether logs are live or historical.

Closes coder/internal#1254

docs/reference/api/tasks.md

@@ -351,7 +351,9 @@ curl -X GET http://coder-server:8080/api/v2/tasks/{user}/{task}/logs \
       "time": "2019-08-24T14:15:22Z",
       "type": "input"
     }
-  ]
+  ],
+  "snapshot": true,
+  "snapshot_at": "string"
 }
 ```