The string-node rename updated display names for discoverability, but
`StringLength` lost the exact legacy `Length` alias. This restores that
alias and adds focused tests for the renamed schema metadata so future
renames keep old search paths intact.
Constraint: Keep scope limited to the recent string-node rename surface
Rejected: Broad schema metadata audit across unrelated nodes | exceeds the changed-area scope for this automation
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: When renaming nodes for search/discoverability, preserve the previous display name as an alias and lock it with a schema test
Tested: uvx pytest tests-unit/comfy_extras_test/nodes_string_test.py
Not-tested: Ruff lint, because uvx temporary download stalled in this environment
