Fix #134: ModuleNotFoundError: No module named 'agents'

[JiwaniZakir]

Closes #134

Defers the ImportError for the missing/wrong agents package from module-load time to call time, preventing ComfyUI from failing to load the entire plugin on startup.

In backend/agent_factory.py, the top-level raise ImportError(...) inside the except block is replaced with a _AGENTS_AVAILABLE = False sentinel. The set_default_openai_api and set_tracing_disabled calls are guarded behind if _AGENTS_AVAILABLE:, and create_agent() now raises the actionable error message only when actually invoked. In backend/service/debug_agent.py, the bare imports of agents.items.ItemHelpers and agents.run.Runner are wrapped in a try/except ImportError that sets both to None, so the module can be imported without a functioning agents package.

• backend/agent_factory.py: _AGENTS_AVAILABLE sentinel, create_agent() guard
• backend/service/debug_agent.py: ItemHelpers, Runner graceful fallback

Verified by installing only the legacy RL agents package (without openai-agents) and confirming ComfyUI loads the plugin successfully, with the error surfaced only upon invoking the debug workflow feature.

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This PR was created with AI assistance (Claude). The changes were reviewed by quality gates and a critic model before submission.

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