ci(changelog): suggest <type>.md when an added file has no type

[datlechin]

Why

• ci(changelog): let towncrier own fragment names and the release section #3514 kept the suffix of a misnamed changelog.d file in the PR title hint. For changelog.d/notes.txt it printed "Name it changelog.d/3600.txt", which towncrier's strict draft in Repo Hygiene then refuses as an invalid fragment name. Before ci(changelog): let towncrier own fragment names and the release section #3514 the hint was 3600.<type>.md.

Change

• The hint keeps the suffix only when it still holds a type and an extension (fixed.md, fixed.2.md). Otherwise it prints changelog.d/<number>.<type>.md.

Verified

• test_a_file_that_is_not_a_fragment_gets_the_type_placeholder fails on main for notes.txt and README.md and passes here. It replaces test_a_file_that_is_not_a_fragment_is_refused, which checked only that the file is refused.