Fixes https://github.com/coder/internal/issues/1371
## Problem
`TestCloseDuringShutdownContextCanceledShouldRetryOnNewReplica` flakes
intermittently in CI. The observed failure is that the chat never
reaches `pending` status after `serverA.Close()`.
## Root cause
Race between context cancellation and the mock OpenAI server's stream
completion marker.
When `Close()` cancels the server context, the in-flight HTTP streaming
request is canceled. The mock server's handler detects this via
`req.Context().Done()` and closes its chunks channel. The mock's
`writeChatCompletionsStreaming` then writes `data: [DONE]` — the SSE
completion marker. On a loopback connection, this marker can reach the
client **before** the client's HTTP transport honors the context
cancellation.
When this happens:
1. The client sees a successful stream completion (not an error)
2. `chatloop.Run` returns `nil`
3. `processChat` falls through without error → status stays `waiting`
(the default)
4. The test expects `pending` → **flake**
## Fix
Skip writing the `[DONE]` marker when the request context is already
canceled, in both `writeChatCompletionsStreaming` and
`writeResponsesAPIStreaming`.
Kyle Carberry