Fix: streaming tool-call arguments dropped when the leading brace ships with the tool name

[anagrius]

Problem

Some OpenAI-compatible servers (llama.cpp, vLLM, …) begin streaming a tool call's arguments in the same chunk as the function name. This is valid per the OpenAI streaming spec — the first delta looks like:

{"index":0,"id":"…","type":"function","function":{"name":"list_metrics","arguments":"{"}}

…and the continuation deltas carry the rest:

{"index":0,"function":{"arguments":"\"database\":\""}}
{"index":0,"function":{"arguments":"dev"}}
…
{"index":0,"function":{"arguments":"\"}"}}

decode_openai_tool_call_delta/1 matches the first chunk on the "complete tool call" clause (name and arguments present), calls build_openai_tool_call_chunk("list_metrics", "{", …), Jason.decode("{") fails, and the "{" is preserved only as :raw_arguments metadata on the tool-call chunk — it is never added to the per-index argument-fragment buffer.

The continuation fragments then accumulate to "database":"dev","table":"dev"} — valid JSON except the leading brace is missing — so message_tool_call_args/2 fails to decode and falls back to the tool-call chunk's own (empty) arguments. The tool ends up being called with %{}.

OpenAI/Anthropic/etc. are unaffected because their first tool-call chunk sends arguments: "" (empty), so every argument byte arrives via the continuation path.

Fix

Seed the per-index fragment buffer with the tool-call chunk's :raw_arguments (the dropped leading fragment) in ChunkAccumulator.push/2 for :tool_call chunks. The continuation fragments then append to it and the joined JSON parses correctly. Providers that send an empty arguments in the name chunk have an empty raw_arguments, so nothing is seeded and their behavior is unchanged.

Test

Added a regression test in chunk_accumulator_test.exs covering the llama.cpp/vLLM shape (first chunk carries "{", continuation fragments carry the rest). Verified it fails without the fix and passes with it; the full ChunkAccumulator suite is green.

[anagrius]

Without this the lib is broken for many custom providers.