fix(openai_chat_completions): don't overwrite tool-call name with an empty-string continuation chunk

[zouxiaoliang]

Summary

Some OpenAI-compatible gateways (observed with Volcengine Ark) send the tool-call function name once on the first streamed delta chunk, then resend it as an empty string ("name":"", not null and not an omitted key) on every subsequent chunk that only carries argument fragments. The streaming tool-call accumulator in openai_chat_completions's stream_text unconditionally overwrites the accumulated name on every Some(name), so the empty-string continuation chunks clobber the real name — by the time the tool call finishes, ToolCallInfo.tool.name is "" and downstream tool dispatch fails with "Tool not found".

• Guard the accumulation with !name.is_empty() so only a non-empty name updates the accumulated value.

Test plan

• cargo check --features openai,openaicompatible passes
• Verified against the real-world case: captured raw SSE from Volcengine Ark for a tool-calling request and confirmed the delta shape (name empty-string on continuation chunks) that triggers the bug
• Applied this patch downstream in a consuming project and confirmed a tool-calling turn against the same Ark endpoint now resolves the tool name correctly end to end