fix(sap-ai-core): normalize finish_reason and strip assistant prefill

[Saichandra2520]

Description

This PR addresses two bugs when using the SAP AI Core provider (sap-ai-core):

1. Claude Models 400 Error (Assistant Message Prefill):
   SAP AI Core Orchestration API rejects messages ending with an assistant message (400: This model does not support assistant message prefill. The conversation must end with a user message.).

   • Fix: In ProviderTransform.message, trailing assistant messages are stripped before sending the request to sap-ai-core.

2. GPT Models Infinite Agent Loop:
   SAP AI Core Orchestration API returns finish_reason: null instead of "stop" in the SSE stream, causing OpenCode's agent loop to not detect completion and loop indefinitely.

   • Fix: Wrapped fetch in provider.ts and SapAICorePlugin to normalize SSE stream payloads ("finish_reason": null -> "finish_reason": "stop").

Related Issues

• Fixes sap-ai-core: Claude models fail with 400 "does not support assistant message prefill" #45313
• Fixes sap-ai-core: GPT models cause infinite agent loop — model never stops responding #45314

Testing

• Added unit tests in packages/opencode/test/provider/transform.test.ts for trailing assistant message stripping.
• Added unit tests in packages/opencode/test/provider/sap-ai-core.test.ts and packages/core/test/plugin/provider-sap-ai-core.test.ts for SSE stream finish_reason normalization.
• Verified all test suites pass.

[github-actions]

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

The search for "finish_reason null stop" found PR #43707 which is about chat streams, but that appears to be unrelated to the SAP AI Core provider issue.

Summary

No duplicate PRs found

The search results only return the current PR #45320 itself (which is correctly excluded from consideration) and one unrelated PR about console chat streams. There are no open PRs addressing the same SAP AI Core provider bugs regarding finish_reason normalization or assistant message prefill stripping.

[github-actions]

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