set gen_specs.user.lb and ub from vocs variables bounds

[jlnav]

Addresses #1621

Claude-written tests

Removes some gen-on-manager mentions.

Most important change: https://github.com/Libensemble/libensemble/pull/1740/changes#diff-6b5d573094ee8401c2efd9058e5137935e1ecbccba7b275bc674ade29d9e3587

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.75%. Comparing base (4e1ab9e) to head (1f03f1a).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
libensemble/specs.py	80.00%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #1740       +/-   ##
============================================
+ Coverage    77.06%   91.75%   +14.68%     
============================================
  Files           79       79               
  Lines         8325     8340       +15     
  Branches      1282     1289        +7     
============================================
+ Hits          6416     7652     +1236     
+ Misses        1665      482     -1183     
+ Partials       244      206       -38     

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