When an LLM API call returns HTTP 4xx with an empty parsed SDK `body` ({}),
`_summarize_api_error` fell through to a bare `str(error)`, so users saw only
"HTTP 400" with no provider detail (reported on Windows in #36109). The SDK
leaves `body` empty in this case, but the httpx `response` still carries the
payload in `.text`.
- run_agent.py `_summarize_api_error`: when `body` is empty, fall back to
`response.text` — parse a JSON `error.message`/`message` when present, else
surface the raw (truncated) body. Platform-agnostic diagnostics.
- hermes_cli/oneshot.py: `hermes -z` now runs via `run_conversation` and returns
exit code 2 when the run is failed/partial with no usable final response, so
scripts can detect LLM failures (still 0 when a response — incl. an error
summary as output — is produced).
Tests: new tests/run_agent/test_summarize_api_error.py (empty-body JSON + raw
text, RED/GREEN verified) + oneshot exit-code/`run_conversation` wiring tests.
NOTE: #36109's original root cause (Windows "all providers return empty 400")
is not reproducible on current main (heavy provider-transport churn since
v0.15.1). This change does not claim to fix that root cause — it makes any
empty-body API error LEGIBLE so a future occurrence shows the real provider
message instead of a bare HTTP 400. Relates to #36109 (does not close it).
