feat: Add OHLC

[ahuang11]

Description

Saw hvPlot had an OHLC plot, which is one of the first non-statistical charts available in hvPlot and not HoloViews, so I decided to ask AI to implement it in HoloViews.

Bokeh

MPL

Plotly (doesn't have a separate edge color for box because of its underlying implementation in plotly)

AI Disclosure

Tool & Model: Opus 4.8
Usage: Provided it context of hvplot and asked to implement across.

• I have tested all AI-generated content in my PR.
• I take responsibility for all AI-generated content in my PR.

Checklist

• Pull request title follows the conventional format
• Tests added and are passing
• Added documentation

[codecov]

Codecov Report

❌ Patch coverage is 98.74214% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.37%. Comparing base (409699b) to head (780eb4e).
⚠️ Report is 28 commits behind head on main.

Files with missing lines	Patch %	Lines
holoviews/plotting/mpl/chart.py	92.59%	4 Missing ⚠️
holoviews/plotting/mixins.py	93.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6913      +/-   ##
==========================================
+ Coverage   89.29%   89.37%   +0.08%     
==========================================
  Files         344      348       +4     
  Lines       74554    75026     +472     
==========================================
+ Hits        66573    67057     +484     
+ Misses       7981     7969      -12     

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