Signup: no email sent for addresses with dots or underscores (API-side)

[Miraeld]

Split out of #1065, which covered two unrelated cases. The + aliasing case is fixed plugin-side in #1259. This one is not a plugin bug.

The case

From #1065:

Case 1: Emails with dots or underscore
Example: film.simleu_21@gmail.com, beatrix.arc.21@gmail.com
=> user is created, but emails are not sent

Signing up with the same addresses directly at https://app.imagify.io/account/sign_up/ works - user created and email sent.

Why this is not plugin-side

. and _ are both valid local-part characters. They are unaffected by:

• the AJAX query-string decoding (unlike +, which decoded to a space - that was Signup fails or emails not sent for certain email formats ("+", ".", "_") #1065's actual bug), and
• sanitize_email(), whose local-part filter (wp-includes/formatting.php:33) explicitly permits . and _.

#1259 adds an integration test, testShouldSendDotsAndUnderscoresToTheApiUnchanged, which asserts that film.simleu_21@gmail.com reaches the API request body unchanged. It passes. The plugin transmits the address correctly.

Since the account is created but no mail arrives, the break is after the plugin: the API's user-creation endpoint or the mail service.

Suggested next step

For the API / infrastructure team, not the plugin team:

1. Create a user via the API with film.simleu_21@gmail.com and check the mail service logs for a send attempt.
2. Compare with the website signup path, which works for the same address - the two must be calling the mailer differently, or one is normalising the address (Gmail dot-folding could make the address look like a duplicate recipient).

Acceptance criteria

• Signing up through the plugin with an address containing dots and/or underscores creates the user and delivers the API key email.
• Behaviour matches the website signup path.