fix(session): keep tool-call arguments as mappings

[atirna]

Issue / context

Fixes #2116

子 agent spawn 之后第一次请求就 400（'int' object has no attribute 'items'），然后这个 session 里换任何模型都一样，直到新开一个。

Type of change

Bug fix

What does this PR do?

SenseNova / 其它 Python OpenAI-compatible 后端会对 tool_call.function.arguments 做 json.loads 再 .items()。tool input 如果是个数字，@ai-sdk/openai-compatible 会 JSON.stringify(0) 发出去变成 "0"，对面 parse 成 int，这段 history 就废了。子 agent spawn 会把 parent 的 inheritedMessages 拼进第一次请求，所以一 spawn 就中。

I coerce primitive/null tool inputs to a mapping in toModelMessages ({} or { value }). ProviderTransform.message also drops stray non-object entries in the messages list and does the same input wrap, since that's the pre-send choke point on the fork path.

How did you verify your code works?

before, on current main: a numeric tool-call input ships as arguments JSON 0 (locked @ai-sdk/openai-compatible@2.0.41).

after:

• toModelMessages turns input 0 into { value: 0 }
• fork-style [...inheritedMessages, ...ownNew, 0] through ProviderTransform.message then a mocked compatible provider: every message is a dict, every function.arguments parses to a non-array object

bun test test/session/message-v2.test.ts test/provider/openai-compatible-messages-schema.test.ts test/provider/transform.test.ts
# 278 pass / 0 fail
bun typecheck
bun lint

Screenshots / recordings

n/a, not a UI change

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR