fix(openai): include reasoning parts without itemId when encrypted_content is present - #12869

Summary

When providerOptions.openai.itemId is absent on a reasoning content part, the convertToOpenAIResponsesInput converter previously skipped the part entirely with:

"Non-OpenAI reasoning parts are not supported. Skipping reasoning part"

This breaks multi-turn conversations for users who remove itemId from their reasoning parts to avoid Item with id 'rs_...' not found errors from the OpenAI Responses API.

Root Cause

The reasoning case in the converter required providerOptions.openai.itemId to be present before including the reasoning in the request. When absent, it fell to the generic warning path — even when reasoningEncryptedContent was available.

Fix

The OpenAI Responses API accepts reasoning items without an id field when encrypted_content is supplied. With this change:

1. If itemId is absent but reasoningEncryptedContent is present → push a reasoning item with only encrypted_content (no id).
2. If neither is present → existing warning is still emitted (no behaviour change).
3. Makes id optional on OpenAIResponsesReasoning type to accurately reflect the API.

Testing

Added two new test cases:

• ✅ Reasoning without itemId but with encrypted_content is included in output
• ✅ Reasoning without either itemId or encrypted_content still warns

Fixes #12853