Add unit tests for hash utils, ID generation, and reranker utils

[fbmz-improving]

Summary

Add unit tests for hashing, ID generation, and reranker utilities.

• test_hash_utils.py (6 tests): covers get_hash determinism, known MD5 vector, uniqueness, unicode input, empty string, and 32-char hex output format
• test_id_generator.py (22 tests): covers create_source_id, create_chunk_id, create_entity_id, create_topic_id, create_statement_id, create_fact_id, rewrite_id_for_tenant, and tenant isolation across all node types.
• test_reranker_utils.py (7 tests): covers to_float numpy scalar coercion and score_values_with_tfidf return type, score ordering, and non-negative score guarantees

Test plan

• Run PYTHONPATH=lexical-graph/src .venv/bin/python -m pytest lexical-graph/tests/unit/ -v — 73 tests pass across all phases
• Verify no regressions in Phase 1 and Phase 2 tests

[acarbonetto]

Please look into the warnings from the run too:

=============================== warnings summary ===============================
src/graphrag_toolkit/lexical_graph/__init__.py:35
  /home/runner/work/graphrag-toolkit/graphrag-toolkit/lexical-graph/src/graphrag_toolkit/lexical_graph/__init__.py:35: DeprecationWarning: There is no current event loop
    loop = asyncio.get_event_loop()

src/graphrag_toolkit/lexical_graph/utils/llm_cache.py:31
  /home/runner/work/graphrag-toolkit/graphrag-toolkit/lexical-graph/src/graphrag_toolkit/lexical_graph/utils/llm_cache.py:31: PydanticDeprecatedSince20: Using extra keyword arguments on `Field` is deprecated and will be removed. Use `json_schema_extra` instead. (Extra keys: 'desc'). Deprecated in Pydantic V2.0 to be removed in V3.0. See Pydantic V2 Migration Guide at https://errors.pydantic.dev/2.12/migration/
    llm:LLM = Field(desc='LLM whose responses may be cached')

src/graphrag_toolkit/lexical_graph/utils/llm_cache.py:32
  /home/runner/work/graphrag-toolkit/graphrag-toolkit/lexical-graph/src/graphrag_toolkit/lexical_graph/utils/llm_cache.py:32: PydanticDeprecatedSince20: Using extra keyword arguments on `Field` is deprecated and will be removed. Use `json_schema_extra` instead. (Extra keys: 'desc'). Deprecated in Pydantic V2.0 to be removed in V3.0. See Pydantic V2 Migration Guide at https://errors.pydantic.dev/2.12/migration/
    enable_cache:Optional[bool] = Field(desc='Whether the cache is enabled or disabled', default=False)

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
======================== 73 passed, 3 warnings in 0.34s ========================