[FEATURE] Improve test coverage: graph builder Cypher generation and VersionManager lifecycle

### Package

lexical-graph

### Problem statement

11 graph builder methods generate Cypher via f-strings. Tests verify call counts but never assert on query string content — structurally invalid Cypher (like #408's `WHERE () AND ...`) passes all tests. `source_graph_builder` has no test file at all. `test_delete_sources.py` and `test_entity_graph_builder_label_injection.py` already assert on query strings — this targets the remaining builders.

`test_version_manager.py` only tests version numbering (get/set/increment/compare). Query generation, source node lookup, lifecycle orchestration, and vector store updates have zero coverage.

Related: #408, #415

### Proposed solution

- Add at least one query-string assertion per builder method (11 total: source, graph_summary, entity, entity_relation, topic, fact, statement, chunk builders + version_manager._get_existing_source_nodes + delete_sources.accept + neo4j_graph_store.init)
- Test VersionManager state transitions: first-time indexing, re-indexing with existing/expired source, accept() lifecycle, error propagation

### Alternatives considered

_No response_