Summary

• merge the fully-populated E2E TOML fixture into the generated template config instead of replacing it wholesale
• preserve template-owned fields when the fixture only needs to override/add app config values
• add lightweight fixture coverage for preserving generated fields and for the Sidekick summary fixture path

Why

PR #7658 fixes the immediate missing [sidekick].extensions_summary field. This stacked follow-up removes the underlying footgun: several E2E tests create an app from the extension-only template, then overwrite the generated shopify.app.toml with packages/e2e/data/valid-app/shopify.app.toml. Future template-required fields could regress the same way.

By treating the fixture as an overlay, tests keep template-provided config unless the fixture explicitly overrides it.

Validation

• pnpm --filter e2e exec playwright test tests/fixture-toml.spec.ts --reporter=list
• pnpm exec tsc --noEmit --project packages/e2e/tsconfig.json
• pnpm --filter e2e lint

Note: pnpm --filter e2e type-check was not useful in this ad-hoc worktree because its Nx dependency builds failed to resolve workspace package dependencies from the symlinked node_modules setup; the direct e2e TypeScript check above passed.