feat(provider-utils)!: align StreamingToolCallTracker with the AI SDK and move it next to its users

[cunninghamcard-bit]

Summary

Now tracker only, based on master; the SSE work moved to its own PR (see #213). Kept as draft until the direction below is confirmed against #185 / ROADMAP.

• StreamingToolCallTracker is rewritten to match @ai-sdk/provider-utils (5.0.51) and moved from aimux-stream to aimux-provider-utils, where upstream keeps it. It emits StreamPart directly (ToolInputStart / ToolInputDelta / ToolInputEnd / ToolCall). Deltas are correlated by wire id, index and function name, ambiguous deltas are dropped, ids are de-duplicated with bounded suffixes, blank function names are ignored, and flush orders calls by index only when every call has one. Adds StreamingToolCallArgumentState / starts_with_structured_value.
• aimux-providers OpenAI chat (openai/model.rs, which serves every registry-backed provider) drives the tracker instead of its private ToolCallAccumulator; DeltaToolCall.index becomes Option<usize> to match the wire. Same wiring as @ai-sdk/openai's chat model.
• aimux-stream no longer exports the tracker or ToolCallStreamPart.

Direction vs. #185 / ROADMAP

#185 and ROADMAP planned "delete the tracker (pure deletion), then introduce ToolInput{Raw,Parsed}". This PR takes the other route and the reasons are:

1. The AI SDK keeps a tracker in @ai-sdk/provider-utils and its OpenAI chat model drives it. aimux's openai/model.rs had its own ToolCallAccumulator doing the same job with index-only correlation, which mishandles reused indices across parallel calls and id-less continuations. Deleting the unused aimux-stream copy would have left that duplicate in place.
2. The Raw/Parsed split that Tool-call input boundary: type the raw/parsed distinction, drop the unused StreamingToolCallTracker #185 wanted is covered by the message-protocol design in docs: full-chain AI SDK alignment design and impact map #200 Part II (V4 tool-call input is the raw string; core's parse_tool_call produces the parsed value). The tracker is orthogonal to it.

ROADMAP's #185 line is updated in #200 (§0.7 table + a note on the line). If the maintainers prefer the deletion route, this PR is closed and only the SSE PR goes in.

Review notes

• CHANGELOG describes only the final state (tracker in aimux-provider-utils, no ToolCallStreamPart); the intermediate return-ToolCallStreamPart API from earlier commits is gone from the entry.
• Tests are ported, not written. streaming_tool_call_tracker_test.rs and the argument-state test are case-for-case ports of the upstream tests at the pinned tag; the tracker test projects StreamPart down to the four fields upstream asserts on.
• Parity was checked by running upstream: the same delta sequences were fed to the JS tracker and the Rust one (identical output for the cases tried). Some odd-looking behaviours are upstream's own (an id-less continuation with a reused index and name is appended to the existing call) and are kept on purpose.
• The default tool-call id generator is the constant tool-call (as before), not a random id.

Checklist

• cargo fmt --all -- --check
• cargo clippy -p aimux-stream -p aimux-provider-utils -p aimux-providers --all-targets -- -D warnings
• cargo doc with RUSTDOCFLAGS=-D warnings
• cargo test -p aimux-stream -p aimux-provider-utils -p aimux-providers
• CI on this head

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https://claude.ai/code/session_01WGwanvo9WLU9HWD7sRR8TS

[eric8810]

这里需要先与 #185 / ROADMAP 对齐 tracker 的方向：原计划是删除 tracker，再引入 ToolInput{Raw,Parsed}；当前最终 diff 是重写、移到 provider-utils，并接入 OpenAI chat stream。两条路线不相同，请明确选择及取舍，再更新相关 issue / roadmap。

SSE 大事件和解析行为修复可以与 tracker 的架构选择分开讨论；如果希望先落可独立验证的修复，可以考虑拆分。

另有两点需要补齐：当前 head 861f26e 没有 CI checks（工作流只匹配 master base）；CHANGELOG 还同时描述返回 ToolCallStreamPart 的中间 API 和最终删除该类型的 API，请统一成最终状态。建议先保持 draft，等方向和当前候选提交的验证都明确后再转 ready。

[cunninghamcard-bit]

已拆分：SSE 部分单独成 PR（见 https://github.com/arcships/aimux/pull/213），本 PR 只剩 tracker，base 改为 master，保持 draft。

方向：选"对齐上游并移入 provider-utils"，不走 #185 的"纯删"。两点理由写在描述里：上游在 @ai-sdk/provider-utils 保留 tracker 并由 OpenAI chat model 驱动，而 aimux 的 openai/model.rs 有一份私有 ToolCallAccumulator 只按 index 关联，并行调用复用 index、无 id 的续片都会出错，删掉 aimux-stream 那份未用的拷贝并不解决它；#185 想要的 Raw/Parsed 区分由 #200 第二部分的四层消息协议承担，与 tracker 正交。ROADMAP 的 #185 行在 #200 里更新。如果维护者仍倾向删除路线，关掉本 PR、只合 SSE 即可。

RFC-0042（#212）的 T2 接受"累积器放 provider-utils"这个归属，T3 要求歧义增量报 InvalidResponseData 而不是像上游那样丢弃——本 PR 现在是上游行为（丢弃）。T3 定下来之前本 PR 不动这一点；若接受 T3，我在这里改成报错并补差分测试，差异登记进允许差异表。T2 里"输出类型最终换成 V4 StreamPart"属于 #200 的类型切换，不在本 PR 范围。

CHANGELOG 改成只描述最终状态（tracker 在 provider-utils、直接产出 StreamPart，没有 ToolCallStreamPart）。

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