chore(state datapoints): Fix state datapoints getitem, slicing and iteration

[haakonvt]

Add proper support for state datapoints (numeric_states, string_states) in DatapointsArray and Datapoints: __getitem__/slicing, & iteration.

Changes:

• Added numeric_states and string_states fields to DatapointsArray initialization and _data_fields()
• Fixed __getitem__() and iteration to:
  • Convert float64 arrays back to int (with NaN -> None)
  • Map plural state keys (numeric_states → numeric_state) for single Datapoint objects
• Fixed dump() to output singular state keys (numericState/numeric_state) in JSON
• Added test coverage for state datapoint getitem, slicing, iteration, and dump behavior

[gemini-code-assist]

Code Review

This pull request adds support for state datapoints (numeric and string states) in Datapoints and DatapointsArray, including plural-to-singular key mapping and handling of missing values (NaN to None). It also adds corresponding unit tests. The feedback suggests optimizing the numpy_dtype_fix function by checking for None early to avoid exception overhead in performance-critical loops.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.16%. Comparing base (08de949) to head (b1bad12).

Additional details and impacted files

@@                           Coverage Diff                            @@
##           add-state-ts-retrieve-arrays-support    #2824      +/-   ##
========================================================================
+ Coverage                                 93.13%   93.16%   +0.03%     
========================================================================
  Files                                       516      516              
  Lines                                     53832    53929      +97     
========================================================================
+ Hits                                      50135    50245     +110     
+ Misses                                     3697     3684      -13     

Files with missing lines	Coverage Δ
cognite/client/data_classes/datapoints.py	90.98% <100.00%> (+0.39%)	⬆️
...ts/tests_unit/test_data_classes/test_datapoints.py	100.00% <100.00%> (ø)

... and 7 files with indirect coverage changes

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