fix(core): continue interrupted responses - #40576

Summary

• preserve output-bearing attempts when a provider response ends unexpectedly
• settle the failed assistant and its tools before scheduling continuation
• append a durable, UI-hidden synthetic user message asking the model to continue without repeating completed content
• write continuation output to a new assistant message while sharing the existing five-attempt retry budget
• retain transparent same-message retries for incomplete streams that produced no output

Testing

• bun typecheck in packages/core
• bun test test/session-runner.test.ts in packages/core (142 pass)
• focused continuation matrix covering text, reasoning, settled tools, projection replay, and retry exhaustion