xAI's grok-imagine-image API returns ephemeral imgen.x.ai/xai-tmp-* URLs
that 404 within minutes — long before downstream consumers (Telegram
send_photo, browser preview, multi-tier delivery fallback) get a chance
to fetch them. The xAI image_gen provider was passing those URLs
through unchanged on the elif url: branch; b64 responses were already
cached locally via save_b64_image. Result: every image_generate call
on a Telegram-routed xai-oauth profile delivered no image, falling
through to text-only.
Adds agent.image_gen_provider.save_url_image() — a sibling helper to
save_b64_image that downloads URL bytes to $HERMES_HOME/cache/images/.
Content-type-aware extension inference with URL-suffix fallback;
oversize cap (25MB default) with partial-write cleanup; empty-body
refusal. Mirrors the audio_cache pattern used by text_to_speech.
Wires save_url_image into both the xAI and OpenAI providers' URL
branches. When the download fails (network blip, 404 in-flight) we
log a warning and fall back to the bare URL rather than turning the
tool call into a hard error — the gateway's existing URL-send fallback
then gets a chance to surface the original error legibly.
Test plan:
- tests/agent/test_save_url_image.py — 8 direct tests against a real
in-process HTTP server: bytes round-trip, content-type → extension,
URL-suffix fallback, default-to-png, 404 propagation, empty-body
refusal, oversize cap + cleanup, filename uniqueness.
- tests/plugins/image_gen/test_xai_provider.py — flip
test_successful_url_response (was asserting the bug), add
test_url_response_falls_back_to_bare_url_when_download_fails.
- tests/plugins/image_gen/test_openai_provider.py — symmetric pair.
160/160 in the broader image_gen test surface.
