* fix(models): replay malformed tool-call arguments as valid JSON
A model that emits a tool call with truncated or otherwise invalid JSON
arguments fails that one call, but the raw string is recorded in the
session. Strict OpenAI-compatible servers (vLLM, SGLang) validate every
assistant tool call in the request, so each later turn is rejected with
"Assistant tool call function.arguments must be valid JSON" and the agent
never recovers. The turn guard now rewrites such arguments on the way out
to a JSON object carrying the original text, alongside the existing
duplicate call-id repair.
* fix(agents): tell the model to re-issue tool calls whose arguments are not valid JSON
A streamed turn that ends mid-tool-call leaves the client with a JSON prefix
as the arguments. Instead of the SDK's generic parse-error result, the tool
wrapper now short-circuits with a clear 'not executed, re-issue with complete
arguments' message so the model recovers in one turn.
* fix(models): treat NaN/Infinity tool-call arguments as malformed
* test(cost): pin model resolution in the provider-prefixed cost test
* fix(models): treat over-nested tool-call arguments as malformed
