fix(render): migrate to validateEmail mutation to restore email existence checks

[Oscar-Quiloulou]

🛠️ Summary

This PR updates the Render provider by replacing the deprecated signUp mutation
with the validateEmail mutation used by Render’s current frontend.
The previous implementation always returned {"email":"invalid"} due to new
server-side validation and mandatory hCaptcha, making the provider unusable.

This patch restores correct behavior for detecting:

• existing emails
• available emails
• invalid email formats

---

🔍 Background

Render recently changed its GraphQL API:

• signUp now requires a valid hCaptcha token
• empty OAuth fields are rejected
• anti-bot validation triggers "email":"invalid" before checking existence

As a result, the old mutation could no longer be used to determine whether an
email is registered.

Render’s frontend now uses the validateEmail mutation, which returns:

{
  "valid": true/false,
  "exists": true/false
}

[kaifcodec]

@Oscar-Quiloulou Thank you for opening this PR.

I tested the module locally, but it currently throws the error: Render Error: GraphQL validation failed.

This is likely happening because some important headers expected by the server are missing. Could you cross-check the request flow again and inspect the actual network request to verify whether any required headers or payload fields are missing?