fix(llm): process terminal SSE frames at EOF

[wolfy-j]

Summary

• complete a pending SSE frame when the transport reaches EOF without the final blank delimiter
• pass terminal response snapshots explicitly into result construction
• cover unterminated response.completed and response.failed events

Root cause

The Responses stream parser retained an unterminated final data line in leftover and returned an empty terminal result at EOF. Bee then rejected the turn because result.response was missing.

Verification

• strict lint: 97 entries, zero issues
• full LLM suite: 1,202/1,202 tests passed