API: enforce sha requirement on POST /repos/{owner}/{repo}/contents (#8139)

Currently the `POST /repos/{owner}/{repo}/contents` API endpoint accepts request without any `ChangeFileOperation.SHA`, unlike stated by the doc:
33eee199cf/modules/structs/repo_file.go (L80-L81)

This PR adds:
- some more (already passing) tests around this function
- a new (failing) test to show this wrong behavior
- a fix (note that this is a breaking change for clients exploiting this bug)
- an update for all the existing tests

<!--start release-notes-assistant-->

## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Breaking bug fixes
  - [PR](https://codeberg.org/forgejo/forgejo/pulls/8139): <!--number 8139 --><!--line 0 --><!--description QVBJOiBlbmZvcmNlIHNoYSByZXF1aXJlbWVudCBvbiBgUE9TVCAvcmVwb3Mve293bmVyfS97cmVwb30vY29udGVudHNg-->API: enforce sha requirement on `POST /repos/{owner}/{repo}/contents`<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8139
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: oliverpool <git@olivier.pfad.fr>
Co-committed-by: oliverpool <git@olivier.pfad.fr>

tests/integration/api_issue_templates_test.go

@@ -47,9 +47,7 @@ func TestAPIIssueTemplateList(t *testing.T) {
 			for _, template := range templateCandidates {
 				t.Run(template, func(t *testing.T) {
 					defer tests.PrintCurrentTest(t)()
-					defer func() {
-						deleteFileInBranch(user, repo, template, repo.DefaultBranch)
-					}()
+					defer deleteFileInBranch(user, repo, template, repo.DefaultBranch)
 
 					err := createOrReplaceFileInBranch(user, repo, template, repo.DefaultBranch,
 						`---