fix(image_gen): soft-accept configured provider in check_fn (#16027)

[Tranquil-Flow]

What does this PR do?

check_image_generation_requirements returned False when:

1. FAL_KEY was not set, AND
2. Plugin discovery hadn't yet registered the provider named in image_gen.provider.

The cached check_fn result determines tool availability for the session's lifetime, so a discovery race at gateway/session init left image_generate permanently excluded from the session — even though _handle_image_generate (which retries discovery with force=True, courtesy of #14502) and the underlying provider both work fine when invoked directly. The user sees the tool simply missing, with no diagnostic.

This PR is the missing companion to #14502: that PR fixed the symptom at call time; this one fixes it at check time. Without both, the tool isn't in the session's tool list to be retried at call time.

Adds a soft-accept gate between the FAL branch and the plugin probe in check_image_generation_requirements: if image_gen.provider is explicitly set in config.yaml (and is not fal, which is the in-tree backend), trust that the user wants this tool and surface it. Provider availability is re-checked at call time in _dispatch_to_plugin_provider, which retries discovery and returns a clear provider_not_registered error if the named plugin still isn't there.

Mirrors the soft-accept pattern used for browser-cdp (#15952) and openai-codex models (#16172).

Related Issue

Fixes #16027

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• tools/image_generation_tool.py: insert a soft-accept gate in check_image_generation_requirements() between the FAL branch and the plugin probe. If _read_configured_image_provider() returns a non-fal value, return True without requiring registry population.
• tests/tools/test_image_generation_check_fn.py (new): 4 tests covering the new behavior plus regression guards.

How to Test

1. scripts/run_tests.sh tests/tools/test_image_generation_check_fn.py tests/tools/test_image_generation_env.py tests/tools/test_image_generation.py tests/tools/test_image_generation_plugin_dispatch.py — 65 pass.
2. Revert tools/image_generation_tool.py change → 2 of 4 new tests fail (the new-behavior tests; the 2 regression guards still pass).
3. Manual repro from the issue: configure image_gen.provider: openai-codex with valid Codex OAuth, restart gateway, send a message — image_generate is now in the session's tool list (previously missing in some sessions due to the race).

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS 15 (Darwin 24.6.0)

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Screenshots / Logs

scripts/run_tests.sh tests/tools/test_image_generation_check_fn.py tests/tools/test_image_generation_env.py tests/tools/test_image_generation.py tests/tools/test_image_generation_plugin_dispatch.py
# 65 pass

[teknium1]

Thanks for the focused regression coverage.

This automated hermes-sweeper review found that the linked #16027 behavior is already implemented on current main:

• tools/image_generation_tool.py:1110-1123 calls _ensure_plugins_discovered() before enumerating and checking registered image-generation providers.
• Commit ff9752410a8dba62f1b246aeed9142893c75b4ba introduced that discovery-before-probe path.
• The linked issue [Bug]: image_generate check_fn returns False at session init due to plugin discovery timing — tool permanently excluded #16027 was subsequently closed as implemented_on_main with the same evidence.

Closing as superseded by the implementation already on main.