fix(python): Consistent expansion of nested struct data during DataFrame init from dict

[alexander-beedie]

Closes #15192.

New test coverage added for this edge-case.

Can follow-up with a further commit that handles nested values inside nested values (the dict expansion is currently constrained to a one-level scalar fast-path that allows us to make use of extend_constant).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.25%. Comparing base (2a3b363) to head (c8aafb5).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15217   +/-   ##
=======================================
  Coverage   81.25%   81.25%           
=======================================
  Files        1354     1355    +1     
  Lines      175631   175653   +22     
  Branches     2518     2520    +2     
=======================================
+ Hits       142702   142734   +32     
+ Misses      32448    32437   -11     
- Partials      481      482    +1     

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