fix(chat-bridge): keep index-less streamed tool-call fragments in one block

[dex0shubham]

Problem

The Chat Completions SSE translator (chatStreamEvent in messages-chat-bridge.ts) keyed streamed tool-call deltas by:

const key = typeof tc.index === 'number' ? tc.index : state.calls.size;

Some OpenAI-compatible upstreams — the arbitrary providers this bridge exists to support (Cloudflare Workers AI, Kimi, etc.) — omit index on tool-call deltas and split a single call's arguments across chunks. With index absent, each continuation fragment was keyed by the growing calls.size, so calls.get(key) missed and a new tool_use block was opened per fragment.

Result for a two-chunk {"q": + "cats"} call: a first block with truncated, invalid JSON plus a spurious second block with an empty name and duplicate id — instead of one tool_use with {"q":"cats"}. The entire streamed-tool-call branch was untested.

Fix

Disambiguate the way the OpenAI stream shape already signals it: a delta carrying an id or name opens a new call; an args-only delta continues the most recent call (tracked via a new lastToolKey on the stream state). Strictly-conformant streams that always send index take the unchanged path.

Added a streaming regression test (index-less, split-arguments) — it opens two blocks on the old code and one on the fix.

Full suite green (1207/1207); typecheck clean.