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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