docs(ui): clarify stream finish message id

[Kage18]

Summary

• clarify that the UI stream message is identified by the preceding start part
• document that messageId belongs on start, not finish

Fixes #8095.

Kage usage

• Used Kage scoped to packages/ai to capture the stream-protocol nuance for future agents.
• Local Kage metrics after refresh: 436 files, 7,411 symbols, 8,863 calls, 2,068 tests, 100% indexed coverage, 3 memory packets, 53 memory graph entities, 54 evidence-backed edges, 0 stale packets.

Tests

• git diff --check

Docs-only change.

[Kage18]

Kage usage for this PR:

This PR fixed #8095 by clarifying the UI stream protocol docs: messageId belongs on the preceding start part, not the finish part.

How Kage helped:

• Scoped recall to the AI SDK UI stream-protocol docs and nearby package context.
• Identified the exact docs file: content/docs/04-ai-sdk-ui/50-stream-protocol.mdx.
• Helped keep the change narrow: clarify the protocol semantics without changing runtime code.
• Captured the reusable nuance that the finish event closes a message whose identity comes from start.
• Captured the docs-only verification path: git diff --check.

Why this matters for future sessions:
A future agent answering stream-protocol questions can recall the start/finish ownership rule and avoid adding incorrect messageId guidance to finish parts.

Full contribution summary and Kage metrics: kage-core/Kage#8