fix: Disable actions on autogenerated docs/nodes

[thseiler]

WHAT:
While working on the image PR, I noticed that it was possible to edit certain node types that StrictDoc cannot currently save:

• autogenerated documents and nodes like test reports
• nodes that were merged from source_code
  These changes appear to take effect initially (as the in-memory model is updated), but they are lost when the server is stopped.

WHY:
Ensures that the user notices that he can currently not edit these node types.

HOW:
The PR introduces an ActionPolicy object and hooks it:

• to the document action button (edit metadata)
• to the node action buttons (edit, clone, delete, add)
• to the toc (move nodes)
• to the individual fields in the edit form

Points for Discussion:

• The ActionPolicy is currently a proposal. I am not really sure whether the policy correctly handles all the edge cases, especially for merged nodes. But as the logic is centralised, it should be easy to adapt as needed.
• I noticed some issues when editing a merge node (where some fields are parsed from the source code and others come from the .sdoc). The origin metadata for fields would be lost on edit, and the UID / MID would not be written back to the sdoc, so that the link would be lost on subsequent server starts. I've tried to address these as well.
• I had some trouble with the SDocTestServer hitting the ReadTimeout, and noticed that the exit handler is not called when an exception happens in the enter block. I added an exception handler to clean up the server so that the end-to-end tests could continue.

[stanislaw]

The PR had a few instances of this. @thseiler I am curious if this is produced by an AI tool. Which tool did you possibly use to create it? I would be curious if we could extend our Dev Guide and let all AI tools avoid creating this pattern in the future.

(I have been using Codex until now and haven't seen this pattern from it)

[stanislaw]

@thseiler I started rebasing your branch and it needed a few changes that led me to editing your commit directly instead of push an additional code review one. The summary of my changes:

• I removed ActionPolicy class. Instead I moved most methods to the DocumentScreenViewObject and here as is_managed_by_source_code. One pattern here I tried to avoid is to add global logic to Jinja templates as that contradicts to the currently adopted pattern of routing everything through the view object objects.
• I did a minor clean up of the new e2e test fixtures, removing some of the unneeded files.

I am still reviewing the impact and consistency of your other changes, expecially the file_traceability_index. and update_requirement.py. Should be done by the end of this weekend.

[stanislaw]

Also, @mettta will review the CSS styles to ensure that this change is consistent with everything else.