StreamingToolArgumentParser

Shared boundary parser for streaming tool-call arguments.

• Empty input yields an empty object sentinel for accumulators.
• Valid JSON is parsed as-is.
• Invalid/partial JSON is preserved as a raw string for later assembly.
• JSON nested more than 512 levels deep is preserved as a raw string too: the arguments are model output, and a value that deep overflows the stack of whatever renders it next (#1562).