fix: reject non-streaming chat on run errors

[jan-kubica]

Summary

• reject non-streaming text calls when the provider stream emits RUN_ERROR
• share the consumer-facing error conversion with StreamProcessor, preserving the provider code and sanitized raw event
• cover the behavior with a unit regression test and a mocked OpenAI wire-level E2E test

Root cause

streamToText accumulated only text chunks and ignored RUN_ERROR. As a result, chat({ stream: false }) could resolve with empty or partial text after a provider failure, while the streaming UI path surfaced the same failure through onError.

Test plan

• pnpm test:pr
• pnpm --filter @tanstack/ai-e2e test:e2e (349 passed, 1 gated live smoke skipped)

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