[Rollout UI] Documentation link title errors are hidden for every link, not just newly added ones

Follow-up to #16971 / #16993.

drop_documentation_link_title_errors in experimenter/nimbus_ui/new/views.py strips title errors from every documentation link in the review errors, not just the blank link that was just added via "+Add link". Since the same mutated field_errors feeds both the Overview card and the sidebar setup issues, a genuine title error becomes invisible everywhere.

Steps to reproduce

1. On a draft rollout, add a documentation link with a valid URL but leave the title unset.
2. Open the rollout detail page.

Actual result

The review serializer reports an error:

[{'title': [ErrorDetail(string='"" is not a valid choice.', code='invalid_choice')]}]

but nothing is shown in the Overview card or the sidebar setup issues. The rollout appears fully set up.

Expected result

A readable error on that link (something like "Please select a link type"), and the issue counted in the setup progress, so the rollout is not sent to review in a state that will be rejected.

Notes

The suppression is only needed for links that are entirely blank — those already show "This field may not be blank." Scoping it to fully-blank rows would keep the #16971 fix and close this gap.

┆Issue is synchronized with this Jira Task

Start in experimenter/nimbus_ui/new/views.py at drop_documentation_link_title_errors and trace how its mutated field_errors feed the Overview card and sidebar setup issues. Reproduce the draft rollout case with a valid URL and unset title, then confirm that fully blank links keep their blank-field suppression while nonblank links retain readable title errors and count toward setup progress.