fix: coerce empty tool-call arguments to {} in Codex Chat transform

No-argument tool calls produced empty argument strings that passed
through both the streaming response path and the request transform
verbatim. Strict OpenAI-compatible upstreams (e.g. Minimax) reject
`arguments: ""` with a 400 "invalid function arguments json string",
while lenient ones (OpenAI, Kimi) silently treat it as an empty object.

Add canonicalize_tool_arguments / canonicalize_tool_arguments_str helpers
that coerce empty/whitespace arguments to "{}" and apply them at the four
tool-call argument sites (streaming finalize + three request/response
transform paths). Leave function_call_output content untouched, where an
empty string is valid.