feat(code): add LocalProvider; fix E2B detection; fix pass-rate Optional import

[kamalraj23]

Summary

Add dev-only LocalProvider for code rewards so contributors can run code_reward locally without E2B/Morph/Piston.
Fix E2B detection to check e2b_code_interpreter import.
Fix Optional import in pass-rate filtering script.

Rationale

LocalProvider unblocks development of code rewards without external services.
E2B detection previously returned False even when installed.
Pass-rate script import prevented a documented workflow from running.

Implementation

• src/open_r1/utils/code_providers.py
• src/open_r1/utils/import_utils.py
• scripts/pass_rate_filtering/compute_pass_rate.py
• tests/test_local_provider.py

Safety

LocalProvider runs code locally and is meant for dev-only use.

Tests

Added unit test verifying correct/incorrect Python snippets score as 1.0/0.0.