fix(ai): classify missing chat finish reasons as incomplete - #44864

Summary

• classify Chat Completions streams that end without a required finish reason as incomplete streams
• allow the existing session runner interrupted-stream recovery path to recognize these failures
• extend the existing Chat compatibility regression test to assert the classification

Test plan

• bun test test/provider/openai-chat.test.ts test/provider/openai-compatible-chat.test.ts from packages/ai
• bun typecheck from packages/ai