Add phonebook contacts-by-group read

[MarcLeinenDE]

Adds client.phonebook.contacts_by_group() using the newly live-confirmed phonebook/getcontactbygroup request shape.

Scope:

• public helper takes integer group, page_capacity, page_index
• exact wire serialization uses strings for all three nested values
• response reuses the existing PhonebookContactsResponse
• validates types/ranges before network access
• no phonebook write in this helper

Upstream:

• protocol evidence normalized in nr2301-api PR Add verified UI language setter #4

Physical validation:

• preliminary direct-call probe passed on ACIY.3 with integer result, integer contactcount, list contactlist
• public high-level helper test test_phonebook_group_read passed 1/1 in 0.46 s
• selected test group returned zero items

Privacy:

• integration test selects a real group index but never prints group names, contact names, phone numbers or contact-list contents

Documentation:

• focused docs/phonebook-group-read.md evidence/usage note added.