docs: import panel in Flask + Bokeh deployment example

[mvanhorn]

Description

The Combining Bokeh Application and Flask Application example in the Deploying Bokeh Apps user guide gives a runnable flaskApp.py that fails the moment Panel renders a HoloViews DynamicMap:

bokeh.core.serialization.DeserializationError: can't resolve type 'panel.models.browser.BrowserInfo'

pn.serve(...) adds Panel-specific models (e.g. panel.models.browser.BrowserInfo) to the document, but flaskApp.py only imports from bokeh.client / bokeh.embed / flask, so when pull_session(...) deserializes the session those types aren't registered with bokeh and the request 500s.

In #6041 philippjfr identified two fixes, including that "the HoloViews docs should be updated to include import panel." Importing panel registers its bokeh model classes so pull_session / server_session can deserialize them on the Flask side. This PR adds that single import panel line to the flaskApp.py block in examples/user_guide/Deploying_Bokeh_Apps.ipynb, with a short comment explaining why the import is there even though the symbol isn't otherwise referenced (so a linter or a curious reader doesn't strip it).

Fixes #6041

How Has This Been Tested?

This is a single import-line addition to a Python code block inside a markdown cell of a .ipynb that the docs build skips (:skip_execute: True in doc/user_guide/Deploying_Bokeh_Apps.rst), so the change has no runtime effect on nbsite/Sphinx output beyond rendering the extra line. I verified:

• python -c "import json; json.load(open('examples/user_guide/Deploying_Bokeh_Apps.ipynb'))" still parses the notebook cleanly.
• The diff is a 2-line insert inside the flaskApp.py markdown code block and does not touch any other cell, executed code, or metadata.

I did not stand up the full Flask + Bokeh + Panel stack locally to re-run the original repro; the maintainer comment on #6041 already established that import panel is the docs-side fix. If you'd prefer the docs to also include import panel.models.browser (the full workaround philippjfr suggested for users hitting the error today), happy to add it in a fixup commit.

AI Disclosure

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

Tools and Models: Claude Code + Opus 4.7. Used to draft the one-line docs change and this PR body; I reviewed the diff and the notebook JSON before pushing.

Checklist

• Pull request title follows the conventional format
• Added documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.22%. Comparing base (4fbc71e) to head (a90bbc3).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6881   +/-   ##
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  Coverage   89.22%   89.22%           
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  Files         340      340           
  Lines       73969    73969           
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  Hits        65999    65999           
  Misses       7970     7970           

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[hoxbro]

Thank you for the PR