fix: enable useLegacyCompletionsEndpoint to use openai package streaming

[uinstinct]

Description

Add supports for useLegacyCompletionsEndpoint to use openai adapter streaming.

closes CON-5285

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• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created

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Summary by cubic

Disables streaming for OpenAI o-series/GPT-5 when the legacy completions endpoint is used, aligning behavior with model constraints. Addresses CON-5285.

^{Written for commit be39317. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 4 files

[RomneyDa]

@uinstinct since stream is already in completion options, how does this impact functionality? I think we should avoid adding a duplicate configuration option.

[uinstinct]

@uinstinct since stream is already in completion options, how does this impact functionality? I think we should avoid adding a duplicate configuration option.

My bad - I thought since completionOptions.stream was removed from config.yaml reference doc, support was dropped.

Added that back to the config.yaml reference in docs