Add Reels pin helpers

[subzeroid]

Summary

• add Client.clip_pin() and Client.clip_unpin() for pinning Reels to the Reels tab/profile Reels grid
• send the Android-observed profile_grid=clips payload to users/{pin,unpin}_timeline_media/
• document the new helpers and add regression coverage for the exact private API payload

Verification

• .venv/bin/python -m pytest tests/regression/test_clip.py tests/regression/test_media.py
• .venv/bin/python -m ruff check instagrapi/__init__.py instagrapi/mixins/clip.py tests/regression/test_clip.py
• git diff --check HEAD~1..HEAD

Capture notes

Verified against Instagram Android 428 traffic with Charon/HAR. Relevant requests returned {"status":"ok"}:

POST /api/v1/users/pin_timeline_media/
{"post_id":"<media_pk>","profile_grid":"clips"}

POST /api/v1/users/unpin_timeline_media/
{"post_id":"<media_pk>","profile_grid":"clips"}

The same profile_grid=clips parameter was also confirmed in the Android APK decompilation for the Reels grid pin/unpin flows.

Fixes #2196