fix(pi-cli): recover assistant text from message updates

[christso]

Closes #922

Summary

• recover assistant content from pi message_update streaming events when agent_end.messages[].content is empty
• keep the existing message_end fallback and add regression coverage for the streamed-delta case

Red

• On main, running a synthetic pi JSON stream with message_update text deltas and an empty agent_end assistant message returned an assistant message with no content
• Repro command used locally:
  bun --eval 'import { _internal } from "./packages/core/src/evaluation/providers/pi-cli.ts"; /* synthetic message_update + empty agent_end */'\n\n## Green\n- The same synthetic stream now returns assistant content 2 + 2 = 4\n- Regression test added in packages/core/test/evaluation/providers/pi-cli-tool-extraction.test.ts\n\n## Verification\n- bun test packages/core/test/evaluation/providers/pi-cli-tool-extraction.test.ts\n- bun test packages/core/test/evaluation/providers/targets.test.ts --test-name-pattern "pi-cli with azure subprovider and base_url"\n- bunx biome check packages/core/src/evaluation/providers/pi-cli.ts packages/core/test/evaluation/providers/pi-cli-tool-extraction.test.ts\n- pre-push hook passed: build, typecheck, lint, test, validate:examples\n

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[christso]

Closing because the reported pi-azure bug did not reproduce as a real Azure E2E failure in this environment. The parser edge case was reproducible, but not the user-facing red path from #922. I verified on main and on this branch with a real Azure-backed eval, and both passed. I’m filing a separate issue for the real Azure failure reproduced on main.