feat(litellm): add image generation provider (#70246)

* feat(litellm): add image generation provider

Registers litellm as an image-generation provider so model refs like
litellm/gpt-image-2 route through the LiteLLM proxy, and
agents.defaults.imageGenerationModel.fallbacks entries of the form
litellm/... resolve without "No image-generation provider registered
for litellm" errors.

Implementation uses the OpenAI-compatible /images/generations and
/images/edits endpoints that LiteLLM proxies for. BaseUrl resolves from
models.providers.litellm.baseUrl (default http://localhost:4000). Private
network is auto-allowed when baseUrl is a loopback/RFC1918 address, which
covers the common self-hosted LiteLLM proxy case without needing
OPENCLAW_PROVIDER_ALLOW_PRIVATE_NETWORK. Public baseUrls keep normal SSRF
defaults.

Default model is gpt-image-2 (matching upstream 4.21+ OpenAI default).
Advertises the same 2K/4K sizes OpenAI now exposes, plus legacy
256/512/1024 for dall-e-3. Supports both generate and edit.

Local patch. LiteLLM has no upstream image-generation support yet; revisit
if upstream adds one.

* ci: rerun after upstream main hot-fix

* fix(litellm): harden image generation provider

---------

Co-authored-by: Chris Zhang <chris@ChrisdeMac-mini.local>
Co-authored-by: Peter Steinberger <steipete@gmail.com>

.github/labeler.yml

@@ -315,6 +315,11 @@
   - changed-files:
       - any-glob-to-any-file:
           - "extensions/lmstudio/**"
+"extensions: litellm":
+  - changed-files:
+      - any-glob-to-any-file:
+          - "extensions/litellm/**"
+          - "docs/providers/litellm.md"
 "extensions: openai":
   - changed-files:
       - any-glob-to-any-file: