Fix sentence-repeater env

[anakin87]

Description

When running vf-eval sentence-repeater -n 1 -r 1, I get the error

...
File ".../verifiers/environments/sentence_repeater/sentence_repeater.py", line 88, in env_response
"content": state["info"]["questions"][state["turn"]],
~~~~~^^^^^^^^
File ".../verifiers/verifiers/types.py", line 115, in getitem
return super().getitem(key)
^^^^^^^^^^^^^^^^^^^^^^^^
KeyError: 'turn'

Replacing state["turn"] with len(state["trajectory"])

@willccbb @mikasenghaas

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
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

Tested env with vf-eval sentence-repeater -n 1 -r 1