[REF] mail: update related_attachment_count field name to match mixin namespace.

Reason: The Attachment count of mail_thread is used by the new
attachment button of the chatter, the compute field has been chosen
over a search_count JS RPC to keep a synchronous chatter and prevent
flickering.

This commit refactors those changes to follow the naming
conventions of mail_thread and to optimize the count method for
eventual reuse follwing TDE's advices.

- Changed the name of a mail_thread's related_attachment_count into
message_attachment_count.

- Changed the count method to a read_group.

PR: 26989
Task: 1860031

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