fix: tool call arguments lost on OpenAI-compatible providers that stream id only in first SSE chunk

[goingforstudying-ctrl]

What

Fixes a bug where tool call arguments are lost (sent as {}) when using OpenAI-compatible providers that omit the id field on tool-call continuation SSE chunks (e.g. GLM-5 via OpenRouter).

Why

Per the OpenAI streaming spec, id is present only in the first delta chunk for a given tool call. Continuation chunks carry argument fragments with the same index but no id.

The current code computes the call-boundary key as:

let maybe_call_id = format!("{}/{}", id.unwrap_or_default(), index.unwrap_or_default());

When id is absent, unwrap_or_default() yields "", producing maybe_call_id = "/0". This does not match the stored call_id, so a spurious boundary is detected: function_arguments is flushed (empty at that point) and cleared. The subsequent argument fragments are appended to the now-empty buffer, but the tool is dispatched with the flushed empty string, normalised to {}.

How

Only advance the boundary when id is present in the chunk — i.e. only on the first chunk of a new call. Continuation chunks have no id and now pass through without touching the boundary logic.

Using {id}/{index} retains uniqueness for providers that reuse index: 0 across every call (the original intent of commit 7d33a2c).

Testing

• Compiled successfully with cargo +nightly check --bin aichat
• Verified the fix handles all three scenarios correctly:
  • Single-chunk call (Grok-4) — unchanged
  • Multi-chunk call, id absent in continuations (GLM-5) — now works
  • Parallel calls, provider reuses index: 0 — still works

Closes #1495

[goingforstudying-ctrl]

Hi, just checking in on this PR. It has been 6 days since the last update. Please let me know if there is anything else needed from my side to move this forward. Thanks for your time.