fix(vend): scope OAuth UID by shop

[nijel]

• Scope Vend social-auth UIDs with domain_prefix to prevent cross-shop numeric ID collisions.
• Validate and propagate domain_prefix through token/user data handling and stored extra data.
• Securely migrate legacy numeric associations only when their saved shop matches the current login.
• Add Vend regression tests and align storage UID typing/test indexes with string UIDs.

[codecov]

Codecov Report

❌ Patch coverage is 89.52381% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.15%. Comparing base (673df79) to head (06fb442).

Files with missing lines	Patch %	Lines
social_core/backends/vend.py	72.22%	5 Missing and 5 partials ⚠️
social_core/tests/models.py	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1795      +/-   ##
==========================================
+ Coverage   83.96%   84.15%   +0.18%     
==========================================
  Files         341      342       +1     
  Lines       12155    12249      +94     
  Branches      579      589      +10     
==========================================
+ Hits        10206    10308     +102     
+ Misses       1742     1728      -14     
- Partials      207      213       +6     

Flag	Coverage Δ
unittests	84.15% <89.52%> (+0.18%)	⬆️

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