fix(notebooks): set name when creating from Markdown

[hippi345]

What does this PR do?

Markdown notebook create now sets a notebook name instead of publishing with an empty name. --name is accepted on pup notebooks create --markdown; when it is omitted, the text of the first ATX heading (#–######, any level) is used. If there is no heading, the name is left empty, matching the previous Markdown create behavior.

The Markdown path writes YAML frontmatter title: (the same key pup's GET-markdown test fixture uses). Only a top-level title: key is replaced; indented / nested title: keys and title: lines inside a block scalar are left alone, and a top-level title: is inserted instead.

Motivation

Fixes #917. pup notebooks create --markdown --file notebook.md stored # Test notebook as cell content and left data.attributes.name as "".

API finding

Proven: The v1 JSON create endpoint (POST /api/v1/notebooks) accepts a required data.attributes.name (NotebookCreateDataAttributes.name: String in the rust client pup pins). Markdown create does not use that JSON body; it POSTs text/markdown to /api/unstable/notebooks.

Proven in-repo: get --markdown is documented as YAML frontmatter + body (docs/COMMANDS.md). The GET-markdown test mocks a server body of ---\ntitle: test\n---\n\n## hi\n (src/commands/notebooks.rs, test_notebooks_get_markdown_requests_markdown_media_type). Pup does not parse that frontmatter; it prints the body as-is.

Inferred, not proven: that POST /api/unstable/notebooks treats YAML title: as data.attributes.name. There is no Datadog public spec for the unstable Markdown endpoint that says so, and this change was not live-round-tripped (no API credentials in this environment).

Before / after

File:

# Test notebook

Hello world!

Before (pup notebooks create --markdown --file notebook.md): posted body is the file as-is; pup notebooks get <id> shows "name": "".

After (same command): posted body is:

---
title: "Test notebook"
---

# Test notebook

Hello world!

--name "Incident notes" sets title: "Incident notes" even when a heading is present. A file with no ATX heading is posted unchanged (empty name).

Additional Notes

• --name requires --markdown (JSON create still takes the name from the request file).
• Setext headings are not used as the default name.
• Headings inside fenced code blocks are ignored.
• Existing frontmatter: only an unindented top-level title: is updated.

Checklist

• The code change follows the project conventions (see CONTRIBUTING.md)
• Tests have been added/updated (if applicable)
• Documentation has been updated (if applicable)
• All CI checks pass
• Code coverage is maintained or improved

Related Issues

Fixes #917

This change was prepared with AI assistance and reviewed by the author.