fix: replace mutable default arguments with None defaults

[dieutx]

Summary

Replace mutable default arguments (dict = {}, list = []) with None defaults in core library files. This is a well-known Python bug where mutable defaults are shared across all calls to a function — if the object is stored as an instance attribute or mutated, it silently corrupts state across unrelated callers.

Changed functions

• Environment.__init__: map_kwargs is stored as self.map_kwargs, meaning all instances using the default share the same dict
• Environment._ensure_prompt, _ensure_task, _format_dataset, _format_completion_dataset: map_kwargs spread via ** — safe today but fragile if any caller later stores or mutates it
• EnvGroup.__init__, _format_dataset, _format_completion_dataset: same pattern as Environment
• StatefulToolEnv.add_tool: args_to_skip is stored in self.skipped_args[tool_name] — all tools added without explicit args_to_skip share the same list
• format_dataset in data_utils.py: map_kwargs spread via **
• OpenAIChatCompletionsTokenClient.tokenize: extra_kwargs spread via **

Pattern applied

# Before
def foo(param: dict = {}):
    ...

# After  
def foo(param: dict | None = None):
    param = param or {}
    ...

Test plan

• ruff check passes on all changed files
• ruff format reports no changes needed
• Full test suite passes: pytest tests/ --ignore=tests/test_envs --ignore=tests/test_envs.py

---

Note

Low Risk
Low risk, behavior-preserving change that prevents shared mutable defaults from leaking state across calls/instances. Minor risk that callers relied on mutating the passed-in default object (now always a fresh dict/list when omitted).

Overview
Fixes several functions that previously used mutable default arguments (e.g. dict = {} / list = []) by switching them to None defaults and initializing locally.

This affects dataset mapping kwargs plumbing (map_kwargs) across Environment, EnvGroup, and format_dataset, tool registration in StatefulToolEnv.add_tool (args_to_skip), and extra request parameters in OpenAIChatCompletionsTokenClient.tokenize (extra_kwargs), preventing accidental state sharing between calls or instances.

^{Written by Cursor Bugbot for commit e9b8054. This will update automatically on new commits. Configure here.}