fix(zen): gate safety identifier by provider config

[Hona]

Summary

• honor the existing provider-level safetyIdentifier flag when building Zen provider options
• stop sending safety_identifier to OpenAI-compatible providers like Kimi K2.5 unless that provider explicitly opts in
• keep the fix minimal by aligning safetyIdentifier with the existing �djustCacheUsage config pattern

[Copilot]

Pull request overview

This PR updates the Zen route provider option-building logic to only include a safetyIdentifier when the selected provider explicitly opts in via its provider config, preventing unintended propagation to OpenAI-compatible providers.

Changes:

• Gate safetyIdentifier assignment behind providerProps.safetyIdentifier.
• Pass undefined for safetyIdentifier when the provider has not opted in, aligning with the existing provider-prop pattern used for adjustCacheUsage.

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