test: improve planner orm and error coverage

[KKould]

Summary

• include the orm feature in coverage runs so src/orm is measured by codecov
• add focused unit tests for error formatting, planner schema/operator behavior, and ORM conversion/helper paths
• cover planner DDL/DML/display operator paths and ORM database/transaction helper APIs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.39%. Comparing base (7cbf9fe) to head (cda9ed5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #370      +/-   ##
==========================================
+ Coverage   88.57%   90.39%   +1.81%     
==========================================
  Files         241      241              
  Lines       39548    39497      -51     
==========================================
+ Hits        35029    35702     +673     
+ Misses       4519     3795     -724     

Flag	Coverage Δ
rust	90.39% <ø> (+1.81%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/errors.rs	98.55% <ø> (+35.74%)	⬆️
src/orm/mod.rs	84.03% <ø> (+10.23%)	⬆️
src/planner/arena.rs	99.68% <ø> (+9.36%)	⬆️
src/planner/mod.rs	95.65% <ø> (+6.52%)	⬆️
src/planner/operator/mod.rs	93.15% <ø> (+56.31%)	⬆️

... and 32 files with indirect coverage changes

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