add VK ID backend

[MeRuslan]

Summary

Adds a new VKIDOAuth2 backend for VK ID under the backend name vk-id.

VK has moved newer integrations to VK ID, which differs from the existing legacy vk-oauth2 backend:

• uses id.vk.ru authorization/token/user-info endpoints
• requires OAuth 2.1 style PKCE with code_challenge / code_verifier
• returns and requires device_id during code exchange
• may return callback data either as flat query params or as a JSON payload

The existing VKOAuth2 backend and vk-oauth2 provider name are left unchanged for backward compatibility.

Docs

• VK ID Web SDK: https://github.com/VKCOM/vkid-web-sdk
• VK ID no-SDK web auth: https://id.vk.ru/about/business/go/docs/ru/vkid/latest/vk-id/connection/start-integration/auth-without-sdk/auth-without-sdk-web

[nijel]

Can you please also add documentation to https://github.com/python-social-auth/social-docs?

[codecov]

Codecov Report

❌ Patch coverage is 86.99187% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.93%. Comparing base (e0c5c02) to head (8f8a3d2).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
social_core/backends/vk.py	79.22%	8 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1743      +/-   ##
==========================================
+ Coverage   83.90%   83.93%   +0.02%     
==========================================
  Files         341      341              
  Lines       12109    12230     +121     
  Branches      575      584       +9     
==========================================
+ Hits        10160    10265     +105     
- Misses       1742     1750       +8     
- Partials      207      215       +8     

Flag	Coverage Δ
unittests	83.93% <86.99%> (+0.02%)	⬆️

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[Copilot]

Pull request overview

Adds a new VKIDOAuth2 OAuth2 backend (vk-id) to support VK’s newer “VK ID” integration (PKCE, device_id, and optional JSON payload callback handling) while keeping the legacy vk-oauth2 backend unchanged.

Changes:

• Introduces VKIDOAuth2 backend with VK ID endpoints, PKCE (code_challenge/code_verifier), device_id handling, and payload callback parsing.
• Adds comprehensive backend tests covering standard callback params and payload-based callbacks.
• Updates CHANGELOG.md to note the new backend.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
social_core/backends/vk.py	Adds the VKIDOAuth2 backend implementation (VK ID endpoints, PKCE, payload parsing, device/user info handling).
social_core/tests/backends/test_vk.py	Adds test coverage for the new backend, including PKCE + device_id assertions and payload callback flow.
CHANGELOG.md	Documents the addition of the VK ID OAuth2 backend.

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