Support Wallet Gateway product navigation

[danielporterda]

Summary

• Updates the Wallet Gateway OpenRPC generator to target the current product-based docs navigation shape.
• Converts the focused Wallet Gateway navigation test from the old dropdown fixture to the current product fixture.

Stack

This is PR 1/7 in the generated-reference automation split.

Base: main
Head: generated-reference-02-wallet-openrpc-nav-products

Validation

• direnv exec . python3 -m pytest tests/test_wallet_kernel_nav.py
• direnv exec . python3 scripts/generate_wallet_gateway_openrpc_reference.py
• direnv exec . python3 -m pytest tests/test_update_generated_reference_sources.py tests/test_update_generated_reference_prs.py tests/test_summarize_version_changes.py tests/test_generate_network_component_versions.py tests/test_wallet_kernel_nav.py tests/test_openrpc.py
• git diff --check

Generator note: the Wallet Gateway generator completed successfully on this branch using the current navigation.products docs shape. It produced generated MDX drift but no docs.json drift; those generated files were reverted so this PR stays scoped to the generator compatibility fix.