Self-hosted - user activation produces 404 when + sign in username

## Bug description

When creating a user account with a **`+` sign** in the username (e.g., `test+api@gmail.com`), the **account activation link: https://{api-name}.developer.azure-api.net/mapi/users/{userId}/identities/Basic/{userEmail}?api-version=2022-04-01-preview** results in a **404 error** when accessed.

## Reproduction steps

1. Go to the **sign-up** page in the developer portal.
2. Create a new user with an email like `test+api@gmail.com`.
3. Submit the registration.
4. Receive the account activation link via email.
5. Fill activation link and headers with proper values from $ConfirmQuery value and send put request using e.g. insomnia
6. Observe that it leads to a **404 Not Found** error page.
## Expected behavior

The activation link should correctly activate user and return 200, even when the email address contains a `+` character.

## Is self-hosted portal?

Yes

## Release tag or commit SHA (if using self-hosted version)

[https://github.com/Azure/api-management-developer-portal/blob/master/src/services/usersService.ts](https://github.com/Azure/api-management-developer-portal/blob/master/src/services/usersService.ts)

## Environment

- **Operating system:** Windows 11
- **Client:** Insomnia 11.0.2

## Additional context

Encoded `+` character along with `@` character produces 404 as well.

Start by reading src/services/usersService.ts and trace how the activation URL is constructed or sent for emails containing a + character. Reproduce the self-hosted flow with an address such as test+api@gmail.com and verify that the activation request returns 200 instead of 404.

Written by the indexing model from the issue text.