add utf encoding in websurfer read file

[victordibia]

Why are these changes needed?

Add utf encoding to file reading.
Without this, a default system encoding will be used. On Windows machines this can default to any local encoding causing errors.

with open(
            os.path.join(os.path.abspath(os.path.dirname(__file__)), "page_script.js"), "rt", encoding="utf-8"
        ) as fh:

Related issue number

Closes #6093

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.75%. Comparing base (7047fb8) to head (aae3537).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6094   +/-   ##
=======================================
  Coverage   76.75%   76.75%           
=======================================
  Files         191      191           
  Lines       13226    13226           
=======================================
  Hits        10152    10152           
  Misses       3074     3074           

Flag	Coverage Δ
unittests	76.75% <100.00%> (ø)

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