fix(genre-tree): fail clearly when a full-list page returns no body

[Andreas-Garcia]

Summary

fetchAllPages crashed with a TypeError in flatMap when any page resolved to null. The fetch wrapper does that when a handleError callback swallows the failure, or on a 204. Now the query rejects with Paginated list page N returned no body, which React Query surfaces as a normal query error.

Changes

• useGenrePlaylist.ts: fetchAllPages wraps each page fetch and throws when the page is null.
• Tests: null first page, null later page, and totalPages: 0.
• CHANGELOG [Unreleased] → Fixed.

Test plan

• pnpm lint && pnpm build && pnpm test (605 pass)

🤖 Generated with Claude Code

[Copilot]

Copilot review overview

🟢 Approval recommended

It is a small, well-targeted null-guard fix whose premise is verified against the fetch-wrapper, with matching test coverage and a correct changelog entry, and no decoupling or export-wiring concerns.

Review effort: Balanced
Findings: None

What changed in this PR

This PR hardens fetchAllPages in the genre-tree module so that useListFullGenrePlaylists fails with a clear, actionable error instead of a TypeError. Previously, when any paginated page resolved to null (which the fetch-wrapper does when a handleError callback swallows a failure, on a 204, or when auth is missing), the subsequent flatMap over response.results threw an opaque TypeError. Now each page fetch is validated and rejects with Paginated list page N returned no body, which React Query surfaces as a normal query error. This improves observability of transient backend/auth failures in both consuming frontends.

Changes:

• Wrap each page fetch in fetchRequiredPage, throwing a descriptive error when a page body is null.
• Add tests for a null first page, a null later page, and the totalPages: 0 short-circuit.
• Add a CHANGELOG.md [Unreleased] → Fixed entry.

File	Description
packages/​app-kit/​src/​genre-tree/​useGenrePlaylist.ts	Adds fetchRequiredPage null-guard so paginated page fetches reject clearly instead of crashing in flatMap.
packages/​app-kit/​src/​genre-tree/​useGenrePlaylist.test.ts	Adds coverage for null first page, null later page, and totalPages: 0.
CHANGELOG.md	Documents the fix under [Unreleased] → Fixed.

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