[Bug] Markdown nested list indentation is stripped on update_object write

Have you read a contributing guide?

• I have read CONTRIBUTING.md
• I have searched the existing issues and didn't find any that were similar
• I have considered creating a pull request with fixes instead of a bug report and want to proceed

Current Behavior

Leading spaces before nested - [ ] items are stripped. The list is flattened:

## Tasks
- [ ] Parent task A
- [ ] Subtask A1
- [ ] Subtask A2
- [ ] Parent task B
- [ ] Subtask B1
- [ ] Subtask B2
  - [ ] Deep subtask B2a   ← only survived because it uses U+2007 figure spaces
- [ ] Parent task C
- [ ] Subtask C1

Expected Behavior

The Tasks section should preserve its nested structure exactly as written.

Steps To Reproduce

1. Read an object that contains nested markdown checkboxes:

## Tasks
- [ ] Parent task A
  - [ ] Subtask A1
  - [ ] Subtask A2
- [ ] Parent task B
  - [ ] Subtask B1
  - [ ] Subtask B2
    - [ ] Deep subtask B2a
- [ ] Parent task C
  - [ ] Subtask C1

2. Update the same object via PUT /v1/spaces/{spaceId}/objects/{objectId} with the same markdown body (only changing a different section, e.g. an Evening note at the bottom).

3. Read the object back.

4. Leading spaces before nested - [ ] items are stripped. The list is flattened:

Environment

- Anytype Desktop: 0.47.x (latest stable)
- API version: 2025-11-08
- Client: @anyproto/anytype-mcp v1.2.9 (but reproducible directly via REST)

Anything else?

• The get_object endpoint does return correct indentation on read (it uses figure spaces U+2007 for nesting in its output).
• However, writing standard markdown spaces ( - [ ]) back through the API causes them to be lost during parsing.
• This makes it impossible to do "surgical" updates to journal/daily-note objects where only the bottom section changes — the whole body must be rewritten, and that rewrite destroys the top-level Tasks hierarchy.
• Workaround: manually using non-standard whitespace (figure spaces) before nested items, but this is not practical for users writing standard markdown.

Start by reproducing the issue through PUT /v1/spaces/{spaceId}/objects/{objectId}, using the nested Markdown checkbox body and then reading the object back. Trace the write-side Markdown parsing and compare it with the get_object output; done means standard-space indentation survives an update and the nested Tasks hierarchy remains intact.