fix(optimiser): handle venues with missing coordinates

[thejus03]

Summary

• Adds a check for venues in venues.json that exist but have missing/zero coordinates (x=0 or y=0)
• These venues are now treated the same as unknown venues by assigning InvalidCoordinates, preventing incorrect distance calculations in the optimiser

Closes #4377

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.39%. Comparing base (988c6fd) to head (8f95f1e).
⚠️ Report is 220 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4378      +/-   ##
==========================================
+ Coverage   54.52%   56.39%   +1.87%     
==========================================
  Files         274      308      +34     
  Lines        6076     6963     +887     
  Branches     1455     1680     +225     
==========================================
+ Hits         3313     3927     +614     
- Misses       2763     3036     +273     

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