feat: support upload note image api

[EastSun5566]

Description

• support for uploading note images via POST /notes/{noteId}/images
• Add uploadNoteImage(noteId, image, options) with multipart FormData support
• Add integration / e2e tests

Manual testing

I manually tested:

• npm run test
• HACKMD_ACCESS_TOKEN=<TOKEN>HACKMD_E2E_MUTATIONS=1 HACKMD_E2E_FOLDERS=0 npm run test:e2e

> @hackmd/api@2.5.0 test:e2e
> jest --config jest.e2e.config.ts

 PASS  tests/e2e/api.e2e.spec.ts
  HackMD API (live e2e)
    read-only
      ✓ getMe returns the current user profile (589 ms)
      ✓ getNoteList returns an array (106 ms)
      ✓ getTeams returns an array (112 ms)
      ✓ getHistory accepts limit and returns an array (216 ms)
      ✓ getFolderList returns folders when the server exposes /folders (223 ms)
    mutations (optional)
      notes CRUD when HACKMD_E2E_MUTATIONS=1
        ✓ createNote creates a note (204 ms)
        ✓ getNote returns the note (209 ms)
        ✓ updateNote updates title, content, and tags (820 ms)
        ✓ uploadNoteImage uploads an image to the note (448 ms)
        ✓ getNoteList includes the note (177 ms)
        ✓ deleteNote removes the note (320 ms)
      folders CRUD when HACKMD_E2E_MUTATIONS=1
        ○ skipped folders: create → get → update → nested folder → list → order round-trip → delete

Test Suites: 1 passed, 1 total
Tests:       1 skipped, 11 passed, 12 total
Snapshots:   0 total
Time:        4.457 s, estimated 7 s
Ran all test suites.