24024 follow-up: fix incorrectly accepting iNaT in validate_fill_value

[jbrockmendel]

remove box_values_as_index

xref #23833, #23982 for overhaul of maybe_promote testing

[jreback]

lgtm. ping on green.

[codecov]

Codecov Report

Merging #24605 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #24605      +/-   ##
==========================================
+ Coverage   92.38%   92.38%   +<.01%     
==========================================
  Files         166      166              
  Lines       52490    52487       -3     
==========================================
- Hits        48493    48491       -2     
+ Misses       3997     3996       -1

Flag	Coverage Δ
#multiple	90.81% <100%> (-0.01%)	⬇️
#single	43.04% <66.66%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/datetimelike.py	98.53% <ø> (-0.31%)	⬇️
pandas/core/dtypes/cast.py	88.72% <100%> (ø)	⬆️
pandas/core/arrays/datetimes.py	98.01% <100%> (ø)	⬆️
pandas/util/testing.py	88.09% <0%> (+0.09%)	⬆️
pandas/core/arrays/timedeltas.py	88.18% <0%> (+0.19%)	⬆️

---

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[codecov]

Codecov Report

Merging #24605 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #24605      +/-   ##
==========================================
- Coverage   92.38%   92.38%   -0.01%     
==========================================
  Files         166      166              
  Lines       52490    52487       -3     
==========================================
- Hits        48493    48490       -3     
  Misses       3997     3997

Flag	Coverage Δ
#multiple	90.81% <100%> (-0.01%)	⬇️
#single	43.05% <33.33%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/indexes/datetimelike.py	98.53% <ø> (-0.31%)	⬇️
pandas/core/dtypes/cast.py	88.72% <100%> (ø)	⬆️
pandas/core/arrays/datetimes.py	98.01% <100%> (ø)	⬆️
pandas/core/arrays/timedeltas.py	88.18% <0%> (+0.19%)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[jreback]

thanks!