Use new subgraph mocking functionality for canned test supergraph

[goto-bus-stop]

Many of our tests use a simple "canned" default schema. This PR replaces the hardcoded query/response mocks with the new subgraph mocking functionality that uses a GraphQL execution engine for that schema.

This is more future-proof in case query plans change in benign ways.

Additionally:

• axum_factory::tests::defer_is_not_buffered was using a query that didn't have existing mocks, but now it works
• Some studio metrics tests were using a query that didn't have existing mocks, so they were always reporting has_errors: true, now it works

[github-actions[bot]]

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[apollo-librarian[bot]]

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Build ID: e8e812a3289c21d20b1c8e9c