plotly: Add missing cast to str for rich.Text conversion.

[daavoo]

DVC passes some values as rich.Text instead of string.
As an oversight in #69 , the refactor to use list_dict_to_dict_list removed this string casting, breaking dvc exp show --pcp

[codecov-commenter]

Codecov Report

Merging #73 (557b811) into main (4fe35d8) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #73      +/-   ##
==========================================
+ Coverage   95.73%   95.79%   +0.05%     
==========================================
  Files          19       19              
  Lines         633      642       +9     
  Branches       92       93       +1     
==========================================
+ Hits          606      615       +9     
  Misses         22       22              
  Partials        5        5              

Impacted Files	Coverage Δ
src/dvc_render/plotly.py	100.00% <100.00%> (ø)
tests/test_parallel_coordinates.py	100.00% <100.00%> (ø)

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