Expand unit test coverage for untested pages

[priscaenoch]

Problem

There are 8 test files under test/ for ~92 source modules in src/. Several core pages have no tests, including ContractPage, EventPage, GraphPage, Sandbox, BatchMultiCall, RpcMetricsDashboard, RateLimitDashboard, and SubInvocationPage.

What needs to be done

• Add Testing Library tests for the untested pages listed above, mocking the API client.
• Cover: initial render, loading state, successful data render, and error state for each page that fetches.
• Add tests for critical utilities in src/utils (formatting, XDR helpers) that currently lack coverage.

Files

• new tests under test/
• may require exporting/mocking the shared API client

Acceptance deliverables

• Each listed page has at least render + loading + error tests.
• Test count meaningfully increases and npm test passes.

Tests to pass

• New tests pass in CI.
• No reduction in existing coverage.