[FEATURE] Improve test coverage: graph_utils boundary cases and Cypher escaping

### Package

lexical-graph

### Problem statement

`graph_utils.py` functions lack boundary and edge-case test coverage. `test_graph_utils.py` covers happy paths; `test_graph_utils_injection.py` covers basic injection patterns (6 tests). The remaining gaps:

- `parse_metadata_filters_recursive`: NOT + empty filters, 3-level nesting, mixed children types
- `escape_cypher_label`: empty string, only-backtick, newline inputs
- `formatter_for_type`: empty string input
- Adversarial escaping: unicode null bytes, homoglyphs, oversized keys (10KB+), newlines in keys, nested backtick+quote combos, values containing Cypher syntax, non-string metadata values

Related: #408, #415

### Proposed solution

Extend `test_graph_utils.py` and `test_graph_utils_injection.py` with the above edge cases.

### Alternatives considered

_No response_