fix(llm): recover missing terminal content

[yaojin3616]

Summary

• retain completed text, reasoning, and tool-call blocks while translating pi-ai streams
• reconstruct a missing terminal message.content array before stop classification and replay persistence
• preserve valid provider terminal messages unchanged

Verification

• patch applies cleanly to the vendored @deepseek-ai/dsh-llm-pi-ai@0.1.2-rc.1 bundle
• patched bundle passes node --check
• focused behavioral assertion covers missing terminal content, content-index ordering, and valid-message identity preservation
• git diff --check

Full Vitest execution was not available because the workspace disk had only about 357 MiB free and npm ci could not complete.

Fixes #379