test: cover lang hash truncation

[txhno]

Description

Adds focused tests for lang_hash in episode_cache.py, covering the 16-byte SHA-256 truncation contract, deterministic output, and uniqueness across a small prompt set.

Closes #79

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
• Infrastructure / CI update
• Test coverage

How Has This Been Tested?

• pytest tests/ passes locally
• reflex doctor sanity check
• Other (please specify):

PYTHONPATH=src pytest tests/test_episode_cache_utils.py -q
ruff check tests/test_episode_cache_utils.py
git diff --check

Targeted pytest result: 3 passed.

Checklist

• My code follows the style guidelines of this project
• 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
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have kept the PR scoped to a single concern (one concern per PR)

[DsThakurRawat]

@rylinjames
@AniketR10
merge this to main