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

[mykola-pereyma]

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