fix: pass both 'openai' and 'azure' providerOptions keys for @ai-sdk/azure

[meruiden]

Issue for this PR

Closes #20275

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When providerOptions() in transform.ts builds model-level options for @ai-sdk/azure, it wraps them under the "azure" key (via sdkKey()). However, @ai-sdk/azure delegates to OpenAIChatLanguageModel from @ai-sdk/openai, which hardcodes provider: 'openai' in parseProviderOptions — so it only reads from providerOptions["openai"]. Model options like reasoningEffort end up under the wrong key and are silently ignored.

The OpenAIResponsesLanguageModel is smarter — it checks "azure" first, then falls back to "openai". So the responses path wasn't affected, only the chat completions path.

Fix: For @ai-sdk/azure, pass model options under both "openai" and "azure" keys. This ensures both the Chat and Responses model implementations pick them up.

This approach avoids changing sdkKey() itself, which is also used for message-level providerOptions remapping (line ~294) where "azure" is the correct key — as addressed in #20326.

How did you verify your code works?

• Configured an Azure OpenAI provider with "options": { "reasoningEffort": "medium" } on a model
• Inspected the outgoing HTTP request and confirmed "reasoning_effort": "medium" is now present in the body
• Verified on latest dev (with fix: rm exclusion of ai-sdk/azure in transform.ts, when we migrated to v6 the ai sdk changed the key for ai-sdk/azure so the exclusion is no longer needed #20326 merged) that the issue still reproduces without this fix
• Typecheck passes (the externalOutputMode error in tui/app.tsx is pre-existing on dev)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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[meruiden]

@rekram1-node Updated the approach based on your feedback on #20275.

The original fix changed sdkKey("@ai-sdk/azure") from "azure" to "openai", but that would conflict with your fix in #20326 — sdkKey is shared between model-level providerOptions() and message-level remapping, and they need different keys ("openai" vs "azure" respectively).

New approach: keep sdkKey returning "azure", and instead have providerOptions() pass model options under both "openai" and "azure" for @ai-sdk/azure. This works because:

• OpenAIChatLanguageModel reads from providerOptions["openai"] ✅
• OpenAIResponsesLanguageModel reads from providerOptions["azure"] first, falls back to "openai" ✅
• Message-level remapping still uses "azure" via sdkKey ✅

Rebased on latest dev with #20326 included. Verified that the issue still reproduces on dev without this fix.