fix: prevent delete failures on empty successful API responses

[cursor]

Summary

• Fixes a critical correctness bug in PixelmuseClient.request where all successful responses were forced through res.json().
• Handles successful empty responses (204/205) by returning undefined instead of throwing a JSON parse error.
• Adds focused client tests covering:
  • deleteGeneration on 204 No Content
  • normal JSON success parsing on getGeneration
  • structured JSON error parsing into ApiError

Bug / Impact

Deleting a generation could fail even when the server successfully deleted it (common REST behavior is 204 No Content).

Root Cause

request<T> always executed return (await res.json()) as T for all 2xx responses. For empty-body success responses, JSON parsing throws (Unexpected end of JSON input), bubbling up as an error and breaking critical delete flows.

Validation

• Installed dependencies with pnpm install
• Ran:
  • pnpm build
  • pnpm test
• Result: all tests pass (including new regression tests).