@effect/ai-openai: Schema.optional not compatible with OpenAI strict mode

[lucas-barake]

Summary

When using Schema.optional for tool parameters with @effect/ai-openai, OpenAI rejects the request with:

Invalid schema for function 'search': In context=(), 'required' is required to be supplied and to be an array including every key in properties. Missing 'limit'.

Root Cause

1. OpenAiLanguageModel.ts:1306 hardcodes strict: true for all tools
2. OpenAI strict mode requires ALL properties to be in the required array
3. Schema.optional excludes the property from required (correct JSON Schema behavior)
4. OpenAI rejects schemas where properties exist but aren't in required

Workaround

Use Schema.NullOr instead of Schema.optional and transform null -> undefined at decode time. Additionally, override the JSON schema annotation to use type: [originalType, "null"].