Refactor handler Fastify internals for clearer route/error flow and safer response assertions

[Copilot]

packages/handler/src/fastify.ts had high local complexity from repeated route bookkeeping and duplicated error-response serialization paths. This change streamlines those internals to improve readability/maintainability while preserving behavior, and adds focused assertions for sanitized error output.

• Route bookkeeping cleanup

  • Extracted route table initialization into a dedicated helper (createDefinedRoutes).
  • Introduced small predicates (hasRoute, hasRouteInAnyMethod) to simplify duplicate route checks and reduce inline branching noise in throwOnDefinedRoute.

• Error response path consolidation

  • Centralized shared Cache-Control: no-store headers.
  • Consolidated repeated 500-response serialization into reusable helpers (stringifyPublicError, sendErrorResponse).
  • Retained explicit auth/tenant branches while reducing duplication in the general error path.

• Targeted test tightening for sanitized responses

  • Extended customErrorHandler coverage with auth and tenant-disabled cases.
  • Switched payload assertions to strict object equality on parsed JSON to verify only public fields are returned (no accidental data leakage).

const NO_STORE_HEADERS = { "Cache-Control": "no-store" } as const;

const sendErrorResponse = (reply: Reply.Interface, status: number, error: PublicErrorShape) => {
    return reply.status(status).headers(NO_STORE_HEADERS).send(stringifyPublicError(error));
};

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