adk docs: confirm or drop "usage lands on the last one" for streaming #1219

Current behavior (verified)

website/content/frameworks/adk.mdx:233 says that when streaming, the callback runs once per partial response and usage lands on the last one.

Offline, a streamed SSE model() / gemini() Runner run whose stream ends with a usage chunk (prompt 3, completion 2, total 5) shows donkey.last_call.total_tokens is None in every callback, including the final one. This holds on google-adk 2.12.0 and 2.13.0, with or without the #1214 bridge, so #1214 didn't cause it.

Proposal

• Find where streamed usage is lost: in the transport's SSE usage capture or in ADK's aggregation.
• Then either fix it with a test, or soften the sentence (§0.3: no unconfirmed behavior stated as fact).

Cross-surface impact

• Nextra docs site: in scope (frameworks/adk.mdx).
• integrations/adk.py, core/transport (streamed usage): maybe in scope, depending on the root cause.
• docs/verified-apis.md: maybe, the §8 ADK rows.
• Other frameworks: check whether the openai-agents streaming docs make a similar claim.
• CLI, registry/, tools/: not affected.
• TypeScript parity: not affected; no TS code exists yet.

Refs #1214.