Fix broad except ValueError in degradation_timeseries_plot for multi-YoY detection

[Copilot]

The multi-YoY duplicate-timestamp case was detected by catching a broad ValueError from .rolling(), which also fires for non-monotonic indexes — masking unrelated errors and potentially failing if the input is unsorted.

Changes

• rdtools/plotting.py — degradation_timeseries_plot:
  • Call results_values.sort_index() before rolling so unsorted datetime indexes never raise ValueError
  • Replace try/except ValueError with explicit results_values.index.has_duplicates check; warning and daily resample only trigger when duplicate timestamps actually exist

# Before: broad exception catch
try:
    roller = results_values.rolling(...)
except ValueError:
    warnings.warn("... multi_yoy ...")
    roller = results_values.resample('D').mean().rolling(...)

# After: explicit duplicate detection
results_values = results_values.sort_index()
if results_values.index.has_duplicates:
    warnings.warn("... multi_yoy ...")
    roller = results_values.resample('D').mean().rolling(...)
else:
    roller = results_values.rolling(...)

Checklist

• Code changes are covered by tests
• Code changes have been evaluated for compatibility/integration with TrendAnalysis
• New functions added to __init__.py
• API.rst is up to date, along with other sphinx docs pages
• Example notebooks are rerun and differences in results scrutinized
• Updated changelog

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.90%. Comparing base (1476f02) to head (ceb41f1).
⚠️ Report is 2 commits behind head on development.

Additional details and impacted files

@@             Coverage Diff              @@
##           development     #490   +/-   ##
============================================
  Coverage        96.90%   96.90%           
============================================
  Files               12       12           
  Lines             2328     2329    +1     
============================================
+ Hits              2256     2257    +1     
  Misses              72       72           

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[cdeline]

LGTM. Tested in the multi-YoY notebook and this explicit check for duplicate indices works.