Fix OpenCodeRLMEnv bugs: task tracking, exception handling, and documentation

[cursor]

Description

Fixed three bugs in the OpenCodeRLMEnv implementation:

1. Task garbage collection prevention: Added _sub_llm_tasks set to track concurrent sub-LLM tasks and prevent premature garbage collection
2. Proper exception propagation: Modified _handle_sub_llm_request to re-raise exceptions (including CancelledError) after handling in the finally block, enabling proper task cancellation
3. Documentation update: Added opencode_rlm_test environment to environments/README.md

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

These fixes follow established patterns from the parent CliAgentEnv class for task tracking and exception handling.