Add a Gutenberg image-block insertion regression fixture

[yoldaolmak]

Problem

The painful editor workflow is not only finding an image; it is placing the image in the correct Gutenberg section without duplicating blocks or disturbing existing content.

Pictovap has WordPress/Gutenberg placement logic. A focused fixture would make this behavior easier for a new contributor to understand and safer to change.

Scope

Add one regression fixture and one test for inserting a Gutenberg image block into existing post content.

A good PR should:

• use synthetic Gutenberg HTML only;
• include at least two headings and one existing paragraph;
• insert one image after the intended heading;
• assert that the generated block contains the expected WordPress media ID, image URL, alt text, and caption;
• assert that running the insertion a second time does not create a duplicate block;
• avoid real WordPress API calls and credentials.

Acceptance criteria

• make contributor-smoke and make contribution-check pass.
• The test proves idempotent image placement for one heading target.
• The fixture is readable enough for future CMS adapter contributors to copy.

Contributor path

This is contribution: no-claim: fork the repo and open a PR directly. No reservation comment is needed.

[yoldaolmak]

Maintainer update: this issue is still open and now has a faster setup check. Before editing the Gutenberg insertion fixture, run:

make install
make contributor-smoke

Then keep the PR focused on one synthetic Gutenberg HTML fixture and one idempotent insertion test. The expected PR should prove that running the placement twice does not duplicate the image block.

[yoldaolmak]

Maintainer update: this issue now has a copy-paste First PR Kit with exact files, focused test command, expected PR size, and boundaries:

https://github.com/yoldaolmak/Pictovap/blob/main/docs/contributing/first-pr-kits.md

No claim comment is needed. Fork the repo, keep the PR focused on this single issue, and paste the focused test output in the PR body.