Import and adapt upstream test suite

[guidefari]

Summary

Import the per-API test files from spotify-web-api-ts and adapt them for Effect semantics.

Context

PRD testing decisions: upstream tests serve as parity benchmark.

Current state: ~16K lines of tests covering auth, request layer, pagination, and browser session. But no per-API-module tests adapted from upstream (e.g. ArtistsApi.test.ts, PlaylistsApi.test.ts). The upstream repo at /Users/guidefari/source/oss/spotify-web-api-ts has these.

Acceptance criteria

• Per-API test files imported and adapted from Promise to Effect
• Observable behavior validated, not internal implementation
• Test coverage for all 12 existing API modules

Related

PRD #9 — user stories 32, 33

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Migrated from guidefari/spotify-effect#45