fix: replace mutable default arguments with None in StatefulToolEnv and MCPEnv

[vominh1919]

Problem

Two instances of the classic Python mutable default argument bug:

1. StatefulToolEnv.add_tool(args_to_skip=[]) — stateful_tool_env.py:69

The default [] is created once at function definition time and shared across all calls. If any caller mutates it (e.g., args_to_skip.append(x)), subsequent calls without an explicit argument see the mutated list. This is a well-known Python footgun.

2. MCPEnv.__init__(mcp_servers=[]) — mcp_env.py:151

Same issue — the default list is shared across all MCPEnv instances that don't pass mcp_servers explicitly.

Fix

Replace =[] with =None and initialize inside the function body:

# Before
def add_tool(self, tool: Callable, args_to_skip: list[str] = []):

# After
def add_tool(self, tool: Callable, args_to_skip: list[str] | None = None):
    if args_to_skip is None:
        args_to_skip = []

Also changed if mcp_servers: to if mcp_servers is not None: to correctly handle an explicitly passed empty list (which is truthy with if mcp_servers: but should still be iterated).

Tests

• Both files pass ast.parse() syntax check
• No behavioral change for callers passing explicit arguments
• Callers passing no argument now get a fresh [] each time instead of a shared one

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Note

Low Risk
Low risk: changes only adjust default parameter initialization to prevent shared mutable state; runtime behavior should be unchanged except eliminating cross-call/instance leakage.

Overview
Prevents Python mutable-default-argument bugs in MCPEnv.__init__ and StatefulToolEnv.add_tool by switching list defaults to None and instantiating fresh lists inside the method.

Also updates the MCP server initialization guard to if mcp_servers is not None so an explicitly provided empty list is handled consistently.

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