llms/openai: add store field support for metadata requests

[pedro-a-n-moreira]

Fixes issue #1484 where llms.WithMetadata causes HTTP 400 from OpenAI: 'The metadata parameter is only allowed when store is enabled'

• Add Store bool field to ChatRequest struct
• Set Store: true automatically when metadata is provided
• Add tests for Store field JSON serialization
• Add test for WithMetadata integration with Store field

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