Summary

Fix GraphQL MCP response parsing for valid GraphQL API responses that omit optional
error metadata. This keeps the tool from replacing actionable API messages with a
Zod invalid_union validation dump.

Fixes #402.

Changes

• Relax the GraphQL error schema so path, locations, and extensions can be
  omitted or null where GraphQL permits it.
• Allow successful GraphQL responses to omit the top-level errors field.
• Add regression coverage for pathless GraphQL errors, path: null errors with
  custom extensions, and successful responses without errors.

Verification

pnpm --filter graphql-mcp-server test
pnpm --filter graphql-mcp-server check:types
pnpm --filter graphql-mcp-server check:lint
pnpm exec prettier apps/graphql/src/tools/graphql.tools.ts apps/graphql/src/tools/graphql.tools.spec.ts apps/graphql/vitest.config.ts apps/graphql/tsconfig.json --check