Defer networkx import for schema generation

[DustyShoe]

Summary

• Defer networkx import in invokeai.app.services.shared.graph until graph functionality actually touches nx.
• Quote nx.DiGraph type annotations so importing the module does not require networkx at annotation evaluation time.
• Add a regression test proving the graph module can be imported when networkx import is unavailable.

Context

PR #9091 exposed an existing CI fragility in the frontend typegen job. scripts/generate_openapi_schema.py imports the FastAPI app, which imports invokeai.app.services.shared.graph, and that module imported networkx at module load time. In CI environments where networkx is not installed, schema generation exits before emitting JSON, then pnpm typegen fails with Unexpected end of JSON input because it receives no schema.

This PR fixes the import-time coupling by deferring the networkx import until graph functionality actually uses it.

Validation

• uv run ../../../scripts/generate_openapi_schema.py | pnpm typegen
• uv run pytest tests/test_imports.py -k graph_module_import_does_not_require_networkx -q
• uv run pytest tests/test_node_graph.py -q
• uv tool run ruff@0.11.2 check invokeai/app/services/shared/graph.py tests/test_imports.py
• uv lock --check
• git diff --check