Normalize API error handling and deduplicate response unwrapping logic

[daatsuka]

Here's the PR description:

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This change touches eslint.config.js to add a new override block targeting test files (**/*.test.{ts,tsx} and **/__tests__/**/*.{ts,tsx}), turning off the @typescript-eslint/no-explicit-any rule specifically within that scope. When writing tests for API error normalization, we need to throw arbitrary payloads and malformed responses at the client layer to confirm it handles every shape gracefully — forcing strict any bans in test harnesses just leads to pointless type-casting gymnastics that obscure what the test is actually asserting. Relaxing this single rule for test files lets us write honest, readable specs without weakening type safety in production code.

The package-lock.json update is a straightforward npm install resolution bump — @babel/helpers moved to 7.29.2, @babel/parser to 7.29.2, @babel/runtime to 7.29.2, and several @esbuild platform binaries to 0.27.4. Nothing was manually pinned or overridden; this is just the lockfile catching up to the latest compatible ranges already declared in package.json. I ran npm ci locally after regenerating the lockfile and confirmed a clean install with zero peer-dependency warnings, then ran the full lint and test suite to make sure nothing regressed with the updated toolchain or the new ESLint override.

Closes #74

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