fix: validate tool args are dict in ToolEnv.env_response

[zamal-db]

While training a tool-calling agent with ToolEnv, I hit a crash mid-rollout caused by a model (Qwen3-4B) producing double-encoded JSON tool arguments. json.loads succeeds but returns a str instead of a dict, which then crashes when unpacked as **kwargs in call_tool.

StatefulToolEnv.env_response already guards against this (lines 141-147), but ToolEnv.env_response was missing the same check. This aligns both methods.

Changes:

• Add isinstance(parsed_args, dict) validation in ToolEnv.env_response, matching the existing StatefulToolEnv pattern
• Non-dict args raise ValueError, caught by the existing error handler (returned to model or stops rollout via stop_errors)
• Add two tests: stop-on-error path and graceful-fallback path

Closes #562

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• Any dependent changes have been merged and published

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Note

Low Risk
Small, localized input-validation change in tool-call parsing plus tests; behavior only differs for malformed/non-dict tool arguments.

Overview
Prevents ToolEnv.env_response from crashing when a model returns double-encoded JSON tool arguments by validating that json.loads(tool_call.arguments) produces a dict and treating non-dicts as a parse error.

Adds coverage for this case with two new tests: one verifying rollouts stop with ToolParseError when ValueError is in stop_errors, and another ensuring the env returns a tool error message and continues when it is not.

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