cherry-pick OIDC changes from gitea (#4724)

These are the three conflicted changes from #4716:

* https://github.com/go-gitea/gitea/pull/31632
* https://github.com/go-gitea/gitea/pull/31688
* https://github.com/go-gitea/gitea/pull/31706

cc @earl-warren; as per discussion on https://github.com/go-gitea/gitea/pull/31632 this involves a small compatibility break (OIDC introspection requests now require a valid client ID and secret, instead of a valid OIDC token)

## Checklist

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### Tests

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [ ] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

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

## Draft release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Breaking features
  - [PR](https://codeberg.org/forgejo/forgejo/pulls/4724): <!--number 4724 --><!--line 0 --><!--description T0lEQyBpbnRlZ3JhdGlvbnMgdGhhdCBQT1NUIHRvIGAvbG9naW4vb2F1dGgvaW50cm9zcGVjdGAgd2l0aG91dCBzZW5kaW5nIEhUVFAgYmFzaWMgYXV0aGVudGljYXRpb24gd2lsbCBub3cgZmFpbCB3aXRoIGEgNDAxIEhUVFAgVW5hdXRob3JpemVkIGVycm9yLiBUbyBmaXggdGhlIGVycm9yLCB0aGUgY2xpZW50IG11c3QgYmVnaW4gc2VuZGluZyBIVFRQIGJhc2ljIGF1dGhlbnRpY2F0aW9uIHdpdGggYSB2YWxpZCBjbGllbnQgSUQgYW5kIHNlY3JldC4gVGhpcyBlbmRwb2ludCB3YXMgcHJldmlvdXNseSBhdXRoZW50aWNhdGVkIHZpYSB0aGUgaW50cm9zcGVjdGlvbiB0b2tlbiBpdHNlbGYsIHdoaWNoIGlzIGxlc3Mgc2VjdXJlLg==-->OIDC integrations that POST to `/login/oauth/introspect` without sending HTTP basic authentication will now fail with a 401 HTTP Unauthorized error. To fix the error, the client must begin sending HTTP basic authentication with a valid client ID and secret. This endpoint was previously authenticated via the introspection token itself, which is less secure.<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/4724
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Shivaram Lingamneni <slingamn@cs.stanford.edu>
Co-committed-by: Shivaram Lingamneni <slingamn@cs.stanford.edu>

modules/base/tool.go

@@ -48,13 +48,10 @@ func BasicAuthDecode(encoded string) (string, string, error) {
 		return "", "", err
 	}
 
-	auth := strings.SplitN(string(s), ":", 2)
-
-	if len(auth) != 2 {
-		return "", "", errors.New("invalid basic authentication")
+	if username, password, ok := strings.Cut(string(s), ":"); ok {
+		return username, password, nil
 	}
-
-	return auth[0], auth[1], nil
+	return "", "", errors.New("invalid basic authentication")
 }
 
 // VerifyTimeLimitCode verify time limit code

modules/base/tool_test.go

@@ -41,6 +41,9 @@ func TestBasicAuthDecode(t *testing.T) {
 
 	_, _, err = BasicAuthDecode("invalid")
 	require.Error(t, err)
+
+	_, _, err = BasicAuthDecode("YWxpY2U=") // "alice", no colon
+	require.Error(t, err)
 }
 
 func TestVerifyTimeLimitCode(t *testing.T) {