Support threaded session creation

[philippjfr]

This PR aims to allow moving the creation of new sessions onto a thread with two primary goals:

1. In the short term this will allow any blocking I/O happening during the creation of a new session to no longer block interactivity on the server.
2. In the long term this is meant to prepare us for a future where free-threaded Python is fully supported.

As-is, using cp314t (i.e. free-threaded Python 3.14) this PR increases the number of sessions created per second by about 25% for a medium complexity app without any blocking I/O.

The approach is simple, we subclass bokeh's ApplicationContext and then reuse the thread pool created by setting --num-threads (in the CLI) and config.nthreads (in Python) and move the session creation onto there.

[codecov]

Codecov Report

❌ Patch coverage is 91.30435% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.10%. Comparing base (ec05ecd) to head (752dabb).

Files with missing lines	Patch %	Lines
panel/io/session.py	90.69%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8407   +/-   ##
=======================================
  Coverage   86.09%   86.10%           
=======================================
  Files         349      349           
  Lines       54821    54863   +42     
=======================================
+ Hits        47199    47238   +39     
- Misses       7622     7625    +3     

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