Recipients who disabled private messages can still be messaged (direct input and role expansion)

Problem/Motivation

Users can opt out of private messages by unchecking "Enable private messages"
on their profile (stored via user.data under privatemsg/enable). The autocomplete matches already filter out such users (PrivatemsgAutocompleteMatcher checks enable), but the form validator (PrivatemsgAutocomplete::validateEntityIdAutocomplete) never checks the flag:

• Direct input: a sender can type Username (123) manually, bypassing the autocomplete, and the validator accepts the user because validateReferenceableEntities() (plain UserSelection) only checks that the entity exists and is referenceable — not the enable flag.
• Role expansion: the privatemsg send to role feature expands a role to all users of the role, including users who disabled private messages. The validator's role-expansion branch adds every role user as a recipient without an enable check.

Result: users who disabled private messages still receive messages and email
notifications, defeating their documented opt-out.

Steps to reproduce

1. User B unchecks "Enable private messages" on their profile.
2. Log in as user A with privatemsg write messages (+ privatemsg send to role for the role path).
3. On /messages/new, type UserB (123) directly (or, if B belongs to a role and the role is in allowed_roles, type RoleName (role)).
4. Submit — the message is delivered to B despite the disabled setting, and B receives the email notification.

Proposed resolution

Enforce the enable setting in the validator, on both paths:

• Role expansion (src/Element/PrivatemsgAutocomplete.php): only add a role
  user to the recipient list when user.data reports enable for them.
• Direct input validation: when looping over the validated entity IDs, reject
  the recipient with a form error if enable is not set (mirroring the existing
  blocked-user check), so a message cannot be created for a disabled user.

The privatemsg_send-relevant checks stay in the validator (the same place the
blocked-user check already lives), so all paths into message creation — direct
typing and role expansion — honor the opt-out.

Remaining tasks

Patch is forthcoming.