`TestAdvisorHappyPath_RootChat` could subscribe after the active test
server had already processed the chat and published transient advisor
deltas, leaving the live delta collector empty.
Use a passive chatd test server until the live subscriber and collector
are registered, then start processing and wait for the expected advisor
deltas before canceling the stream.
Closescoder/internal#1548
Generated by Coder Agents.
<details>
<summary>Implementation notes</summary>
The failing assertion covered stream-only advisor `ResultDelta` events.
`CreateChat` signals the processor, so an already-started server can
publish those deltas before `Subscribe` registers its local stream
subscriber. The test now creates the chat on a passive server,
subscribes, starts the collector, then calls `Start()`.
</details>
Michael Suchacz
4d3bfa5fab