fix(ai): tolerate empty-string tool call id/name in streaming deltas

[Garfier]

Problem

Some OpenAI-compatible APIs (e.g. DashScope token-plan / GLM-5.2) send empty strings for id and function.name on subsequent tool call deltas instead of omitting them or sending null:

delta 1: {"id":"call_8c1f...","function":{"name":"calculator","arguments":"{"}}   ✅
delta 2: {"id":"","function":{"name":"","arguments":"\"expression\": "}}          ⚠️ empty string
delta 3: {"id":"","function":{"name":"","arguments":"\"2"}}                       ⚠️ empty string

ToolStream.appendOrStart uses ?? (nullish coalescing) to fall back to accumulated values, but "" is not null/undefined so the fallback never triggers. The subsequent !id check (!"" → true) then fails with:

OpenAI Chat tool call delta is missing id or name

Fix

Treat empty strings as absent in appendOrStart so the accumulated identity from the first delta is preserved:

- const id = delta.id ?? current?.id
- const name = delta.name ?? current?.name
+ const id = (delta.id || undefined) ?? current?.id
+ const name = (delta.name || undefined) ?? current?.name

Testing

Added test case "tolerates empty-string id and name on subsequent deltas" simulating the DashScope token-plan streaming format. All 8 tests pass.

Verified against the real token-plan API (token-plan.cn-beijing.maas.aliyuncs.com) with GLM-5.2.

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