fix: add tools directly to Overlay

[Azaya89]

fixes #6240

This PR fixes the problem where tools specified via .opts(tools=[...]) on an Overlay were being ignored. The OverlayPlot._init_tools() method only collected tools from subplots but never processed tools from self.default_tools and self.tools.

Solution

• Added logic to iterate through self.default_tools + self.tools and add them to the plot
• Improved tool deduplication: Replaced simple type-based deduplication (tool_types list) with unique identifier-based deduplication (tool_ids set). This is better since most bokeh tools like PanTool have variants differentiated only by their dimensions (e.g., 'pan', 'xpan', 'ypan' are all of type PanTool)
• Preserved special handling for hover tools and subcoordinate_y zoom tools which was causing the initial test failures
• Added tests

import holoviews as hv
hv.extension('bokeh')

overlay = (hv.Curve([1, 2, 3]).opts(tools=['xpan']) * hv.Curve([2, 3, 4]).opts(tools=['ypan', 'zoom_out'])).opts(tools=['zoom_in', 'xpan', 'ypan'])
overlay

[Azaya89]

This works now but it seems a bit verbose to me. Will try to find a more elegant way to implement...

[codecov]

Codecov Report

❌ Patch coverage is 99.31034% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.22%. Comparing base (c4ee605) to head (7cdb736).

Files with missing lines	Patch %	Lines
holoviews/plotting/bokeh/element.py	97.95%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6713      +/-   ##
==========================================
+ Coverage   89.20%   89.22%   +0.01%     
==========================================
  Files         332      332              
  Lines       72010    72135     +125     
==========================================
+ Hits        64240    64364     +124     
- Misses       7770     7771       +1     

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