tests(agent): cover the OpenAI batch request bound

[AdamMagued]

Resolves #176

Introduces an idle timeout on OpenAI-compatible stream reads that resets on each received line. This prevents endpoints that hold connections open without emitting [DONE] or closing from stalling indefinitely while preserving support for late usage chunks.

[yusiwen]

Thanks @AdamMagued — the diagnosis was right, and the fix landed separately while this PR was open:
#178 (rebase-merged as 49964e2) added the idle bound,
the per-line watchdog reset and the named cancel cause — the code half of #176, which is now closed.

What this diff still added is TestOpenAIBatchTimesOutOnSilentEndpoint, which master lacked:
openAIRequestTimeout had no test at all, while its Ollama counterpart
(TestOllamaBatchTimesOutOnSilentEndpoint) did.

Landing: squash-merged as a6b86c8 with the subject reworded to tests(agent): … (the change is
test-only) rather than rebase-merging 7222e40, whose message describes code that is already in master.
The merged commit is attributed to your account.

Two follow-ups found while reviewing this area, both filed with measured repros:

• #185 — a healthy long stream is still cut by the
  client-wide 120 s timeout (http.Client.Timeout covers the body read; Ollama's client has no timeout).
• #186 — the idle bound cannot fire under the default
  settings, because the request bound is the same 120 s and the watchdog is reset by every line, so the
  reader gets Client.Timeout exceeded … instead of stream went silent.

Both are open and unassigned. FYI, minor: your own commit 7222e40 carries adammaged8@gmail.com,
which is not connected to your GitHub account, so commits authored with that address do not link to your
profile — the squash commit above used your account identity, so this one is already credited.