[TST] HealthCheck failure fix in test_ecdf_string

[mjsr84]

Add @settings(suppress_health_check=[HealthCheck.too_slow], deadline=None)
to test_ecdf_string. Without it, Hypothesis raises FailedHealthCheck
because series(dtype=object) is slow to generate. Also imports HealthCheck.

PR Description

Please describe the changes proposed in the pull request:

• 'test_ecdf_string' was missing a '@settings' decorator, causing Hypothesis to raise FailedHealthCheck: Input generation is slow'
• 'series(dtype=object)' is slow to generate without 'suppress_health_check=[HealthCheck.too_slow]', Hypothesis aborts before running any examples
• Fix matches the pattern already used in 'test_ecdf' in the same file
• Root cause: Dec 2025 commit that edited 'test_ecdf' for dtype changes presumably accidentally dropped the decorator from 'test_ecdf_string'

This PR resolves unreported issue.

PR Checklist

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Automatic checks

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• Making sure that code coverage doesn't go down.

Relevant Reviewers

Please tag maintainers to review.

• @ericmjl

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.76%. Comparing base (901f4b3) to head (4dbc2d0).
⚠️ Report is 183 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1609      +/-   ##
==========================================
- Coverage   87.56%   85.76%   -1.80%     
==========================================
  Files          95      125      +30     
  Lines        6819     9892    +3073     
==========================================
+ Hits         5971     8484    +2513     
- Misses        848     1408     +560     

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