tests: add introspection edge case (refs #628)

add introspection edge case coverage and clarify mixed query behaviour in docs (refs #628) ## What kind of change does this PR introduce? **Tests added** (test/sql/introspection.sql): - off/on schema combination (public=OFF, private=ON) — pins the \`any()\` logic from both directions - Named fragment spread containing \`__schema\`/\`__type\` when disabled - Mixed query (introspection + data field) when disabled — \`data\` becomes \`null\` **Docs fix** (docs/configuration.md): - The \"non-introspection queries are not affected\" statement was misleading for mixed queries. Added a warning admonition with an example showing the \`data: null\` behaviour.