fix: always return ModelSettings from _build_model_settings

[jsonbailey]

Summary

• Remove the if kwargs else None guard in OpenAIAgentRunner._build_model_settings() so an empty ModelSettings() is returned instead of None when no parameters are set.

Test plan

• Verify OpenAIAgentRunner works correctly when no model parameters are configured

🤖 Generated with Claude Code

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Note

Low Risk
Low risk, localized behavior change that removes a None return path; only potential impact is if downstream code relied on model_settings=None semantics in the Agents SDK.

Overview
Ensures OpenAIAgentRunner._build_model_settings() always constructs and returns ModelSettings(**kwargs) instead of returning None when no known model parameters are set, so the agent is consistently initialized with a ModelSettings object.

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