fix(image-gen): honor top-level image_gen.model and dispatch model kwarg in the xAI provider

[hashbender]

What does this PR do?

Fixes a parity gap in image-gen model resolution. hermes tools persists the
selected image model to the top-level image_gen.model, and the
image_generate dispatcher forwards it to provider.generate() as a model
kwarg. The xAI provider read only the scoped image_gen.xai.model (plus
the XAI_IMAGE_MODEL env var) and called _resolve_model() with no arguments
— so a user's hermes tools selection was silently dropped and xAI always fell
back to its default grok-imagine-image.

This is the same gap recently fixed for the OpenRouter/Nous provider; openai,
openai-codex and krea already honor the top-level image_gen.model, so xAI was
the lone holdout. This brings xAI in line with the rest.

Related Issue

N/A

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• plugins/image_gen/xai/__init__.py
  • Add _load_image_gen_config() — reads the top-level image_gen section.
  • _resolve_model(explicit=None) now follows the same precedence as the
    other providers: explicit dispatch model kwarg → XAI_IMAGE_MODEL env →
    scoped image_gen.xai.model → top-level image_gen.model → DEFAULT_MODEL.
  • generate() forwards the dispatched kwarg: _resolve_model(kwargs.get("model")).
  • The scoped reader _load_xai_config and _resolve_resolution are left untouched.
• tests/plugins/image_gen/test_xai_provider.py — add scoped / top-level /
  explicit-kwarg model-resolution coverage.

How to Test

Reproduction: set the image model in the top-level image_gen.model (what
hermes tools writes) without a scoped image_gen.xai.model, then resolve the
xAI model.

• Before: plugins.image_gen.xai._resolve_model() returns grok-imagine-image
  (the default) — the selection is ignored.
• After: it returns the selected model. The scoped key still works, and an
  explicit dispatched model kwarg wins over config.

Tests run and results:

pytest tests/plugins/image_gen/test_xai_provider.py::TestConfig -v

  TestConfig::test_default_model ........................... PASSED
  TestConfig::test_default_resolution ...................... PASSED
  TestConfig::test_custom_model ............................ PASSED
  TestConfig::test_scoped_config_model            (new) .... PASSED
  TestConfig::test_top_level_config_model         (new) .... PASSED
  TestConfig::test_explicit_model_kwarg_wins_over_config (new)  PASSED
  => 6 passed

The change is isolated to xAI model resolution; verified against an unmodified
baseline that it introduces no new failures in the full
tests/plugins/image_gen/ suite (it adds the 3 passing tests above).

Checklist

• Read the Contributing Guide; commit follows Conventional Commits (fix(image-gen): …)
• Searched existing PRs/issues to avoid a duplicate
• PR contains only changes related to this fix
• Ran the tests for the affected area — pass (results above)
• Added tests for the change
• Documentation / config / tool-schema — N/A (no new config keys; image_gen.model is already documented and honored by the other providers)

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Mirror-of: NousResearch#55927
NousResearch#55927