Fix image_generate per-call provider/model overrides

[lkz-de]

Summary

• add per-call provider / model override support for image_generate and thread those overrides through the FAL path end-to-end
• warn and fall back to the default FAL model when an explicit per-call FAL model override is unknown, instead of hard-erroring the request
• keep MoA manual and session-scoped by blocking persisted default/main-model assignments to provider: moa
• preserve /moa <prompt> one-shot behavior while stopping implicit or durable default switching into MoA
• document the optional provider / model override args and preserve the image_generate response-field wording around the returned image

Why

On current origin/main, image_generate still lacks the full per-call override flow and the branch also needs follow-up policy enforcement around FAL fallback behavior and MoA persistence.

This PR lands the override support and the follow-up policy fixes together:

• explicit FAL model overrides now survive the plugin path and degrade safely with warn + fallback when the named model is unknown
• MoA remains available for deliberate manual interactive use, but is no longer allowed to become the persisted default/main route

What changed

Image generation / FAL

• add the per-call provider / model override surface and thread it through image_generate
• change _resolve_fal_model(...) so unknown explicit per-call FAL model overrides emit a warning and fall back to the default FAL model
• forward explicit model overrides through plugins/image_gen/fal/__init__.py
• add regression coverage for the fallback behavior and plugin-path forwarding
• document the optional override args while preserving the correct image_generate response-field wording

MoA policy

• reject persisted default/main-model assignments where provider == "moa"
• make explicit interactive MoA switches session-scoped in the CLI/TUI model-picker path
• stop treating bare preset-name input as an implicit switch into MoA defaults
• preserve /moa <prompt> as a one-shot path by leaving that path available while preventing default-routing changes into MoA

Verification

• pytest -q tests/hermes_cli/test_model_switch_moa_policy.py tests/hermes_cli/test_web_server_moa_policy.py tests/tui_gateway/test_goal_command.py::test_moa_bare_returns_usage tests/tui_gateway/test_goal_command.py::test_moa_arg_is_always_one_shot -o addopts=''
  • result: 6 passed
• pytest -q tests/tools/test_image_generation.py tests/tools/test_image_generation_plugin_dispatch.py -k 'fal or override or model' -o addopts=''
  • result: 32 passed, 39 deselected
• python3 -m py_compile cli.py hermes_cli/model_switch.py hermes_cli/web_server.py plugins/image_gen/fal/__init__.py tools/image_generation_tool.py tui_gateway/server.py tests/hermes_cli/test_model_switch_moa_policy.py tests/hermes_cli/test_web_server_moa_policy.py tests/tools/test_image_generation.py tests/tools/test_image_generation_plugin_dispatch.py
  • result: success

Notes

• full CLI MoA collection coverage was not exercised in this environment, so verification here uses the targeted policy and gateway subsets above

[lkz-de]

Splitting this into two narrower follow-up PRs for cleaner review:

• fix(image_generate): honor per-call provider/model overrides and warn-fallback unknown FAL overrides #59815 — image_generate per-call provider/model overrides and unknown explicit FAL override warn+fallback behavior
• fix(moa): keep MoA manual, session-scoped, and non-persistent #59816 — keep MoA manual/session-scoped/non-persistent, including the null-guard regression fix surfaced by CI

I’m closing this combined PR in favor of those two split PRs so each lane can be reviewed independently.