fix(image-gen): honor image_gen.model for Nous/OpenRouter provider

[kshitijk4poor]

Summary

The OpenRouter/Nous image-gen provider now honors the model you actually selected — both the top-level image_gen.model config key (written by hermes tools) and an explicit model kwarg — instead of silently ignoring them and always running the quality-first default chain (gpt-5.4-image-2 → gemini-3-pro-image).

Root cause: _resolve_model_chain() only read the scoped image_gen.<provider>.model key, while hermes tools writes the user's pick to top-level image_gen.model and the image_generate dispatch passes it as a model kwarg — both were dropped. This made OpenRouter/Nous the lone outlier among the multi-model image-gen plugins (krea/openai/openai-codex/xai already read top-level image_gen.model).

Changes

• plugins/image_gen/openrouter/__init__.py: _resolve_model{,_chain} accept an explicit override (the model kwarg) and resolve top-level image_gen.model, matching the established sibling precedence: explicit kwarg → *_IMAGE_MODEL env → scoped image_gen.<provider>.model → top-level image_gen.model → default chain.
• Docstrings aligned to the new precedence (follow-up commit).
• Tests: top-level config override, Nous honoring top-level, explicit-kwarg-wins, and generate(model=...) end-to-end.

Validation

Config	Before (main)	After
image_gen.model: openai/gpt-image-2	[gpt-5.4-image-2, gemini-3-pro-image] (ignored)	[openai/gpt-image-2]
explicit model kwarg	dropped	wins over config
no config	default chain	default chain (unchanged)

• tests/plugins/image_gen/ — 167 passed.
• E2E with real config.yaml + real config loader (temp HERMES_HOME): 6/6 assertions pass (both nous + openrouter honor top-level model; explicit kwarg overrides; no-config falls back correctly).
• ruff + ty: clean.

Credit

Based on #55655 by @xxxigm. Cherry-picked to preserve authorship; docstring alignment added on top.

Closes #55655