SAML sign-in now only links an existing user when that user already
belongs to the account that initiated SSO. New users can still be
created for SAML-enabled accounts, and invited members can continue to
sign in through their IdP, but SAML will no longer auto-attach an
unrelated existing user record during login.
**What changed**
- Added an account-membership check before SAML reuses an existing user
by email.
- Kept first-time SAML user creation unchanged for valid new users.
- Added builder and request specs covering the allowed and rejected
login paths.
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