fix(raycast): surface API error details

[vivekvar-dl]

Summary

The Raycast client only read message from JSON error responses, while Supermemory APIs commonly return { "error": "..." }. Users therefore saw a generic HTTP status instead of the actionable server response.

This change:

• reads the standard error field, with message as a fallback
• preserves useful plain-text error responses
• keeps the existing status-text fallback for empty or unrecognized bodies
• adds focused parser coverage

Verification

• bun test apps/raycast-extension/src/api-error.test.ts (7 passed)
• npm run build in apps/raycast-extension
• npx tsc --noEmit -p tsconfig.json after Raycast generated its manifest types
• git diff --check

npm run lint still reports the repository's existing Prettier mismatch across every Raycast source file; ESLint and manifest validation complete before that pre-existing failure.

Repository-wide checks

• bun run format-lint — blocked by existing repository diagnostics (3 errors and 104 warnings, including Biome schema-version mismatches); no fixes were applied
• bun run check-types — blocked in the unchanged packages/ui/components/shadcn-io/dropzone.tsx because react-dropzone is not declared/installed
• bun run build — all 7 build tasks completed successfully; the root command then failed in postbuild because the Sentry upload requires a maintainer auth token