When the in-tree FAL path has no API key (and no managed gateway), the
handler used to return a bare 'FAL_KEY environment variable not set'
error. Users had no idea where to get a key, that a managed Nous
gateway exists, or that plugin-registered providers are an option.
Now `image_generate_tool` returns a structured multi-line message:
- signup link (https://fal.ai)
- managed-gateway status (if Nous tools are enabled)
- pointer to `hermes tools` / `hermes plugins list` for alternate
backends, so users on a stale `image_gen.provider` know where to look
The schema is untouched — `check_fn` still gates the tool out of the
schema when no backend is reachable at startup, consistent with every
other conditional tool. This patch fixes the call-time failure modes:
managed-gateway 5xx, plugin provider disappearing mid-session, etc.
Inspired by #2546 / @Mibayy. The PR was ~5700 commits stale against
the new plugin-aware image_gen architecture, so this is a forward port
of the actionable-error idea rather than a cherry-pick.
Closes#2543
Co-authored-by: Mibayy <mibayy@users.noreply.github.com>
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