Pin down how query answers a path-scoped GitHub error

[Thayorns]

graphql-client stamps a not-found error like path: ["repository"] as normalizedPath: ["data", "repository"] once data itself is non-null, and result.errors only surfaces errors whose normalized path has no extra segment. That is what this issue was about, and #672 has since fixed it on master: query now folds result.data.errors into the same message list, so a renamed or deleted repository raises Fbe::Error instead of returning a stray nil.

Since the production fix already landed, this branch keeps only what was missing around it. Three tests now pin the behavior down: a path-scoped error arriving together with partial data still raises rather than handing the caller a half-filled result, a top-level error is not swallowed when a path-scoped one sits beside it, and a null data with no errors at all is left alone instead of blowing up on data.errors.

Closes #655

[Thayorns]

@yegor256 happy to clarify anything about this fix if needed.

[yegor256]

This looks like a near-duplicate fix of the same result.data.errors gap already merged via #672 — please check whether it's still needed. Assigning to @yegor256.

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[Thayorns]

@yegor256 take a look please