Standardize PPC error handling by replacing stop() with rlang abort()

[utkarshpawade]

Fixes #455

This PR replaces the remaining base stop() calls in PPC code paths with rlang::abort() to align with existing package conventions for structured error handling.

What changed

• Updated validation errors in ppc_km_overlay() to use abort()
• Updated interpolation validation errors in get_interpolation_values() to use abort()

Why

bayesplot already uses abort() in most places. This change removes inconsistency in error handling and improves maintainability without changing intended functionality.

[Copilot]

Pull request overview

This PR standardizes error handling in the PPC censoring and interpolation helpers by replacing the last remaining base stop() validations with rlang::abort(), aligning these code paths with the package’s prevailing structured error approach (Fixes #455).

Changes:

• Replaced stop() with abort() for input validation in ppc_km_overlay().
• Replaced stop() with abort() for interpolation feasibility checks in get_interpolation_values().
• Documented the consistency change in NEWS.md.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
R/ppc-censoring.R	Switches ppc_km_overlay() validation errors from stop() to abort().
R/helpers-ppc.R	Switches interpolation validation errors in get_interpolation_values() from stop() to abort().
NEWS.md	Adds a development NEWS entry describing the standardized error handling.

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[jgabry]

Thanks, this looks good. Once #460 is merged we can update this branch and rerun the tests before merging.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.55%. Comparing base (b1cb851) to head (95aa7cc).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #456   +/-   ##
=======================================
  Coverage   98.55%   98.55%           
=======================================
  Files          35       35           
  Lines        5864     5864           
=======================================
  Hits         5779     5779           
  Misses         85       85           

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[utkarshpawade]

Thanks!