Fix flake8 linting errors in build_notebooks.py

[Copilot]

CI workflows were failing due to PEP 8 violations in scripts/build_notebooks.py.

Changes

• Removed unused import: os module was imported but never referenced
• Fixed line length violations: Split lines exceeding 79 characters
  • Wrapped long docstring lines
  • Multi-line conditional with proper indentation
  • Refactored HTML error string into multi-line variable
• Added required blank lines: 2 blank lines between top-level function definitions per PEP 8
• Improved consistency: Replaced .format() with f-string, fixed operator spacing

# Before
import os  # unused
if ".ipynb_checkpoints" in p.parts or "site" in p.parts or "gh-pages" in p.parts:
    ...

# After  
if (".ipynb_checkpoints" in p.parts or "site" in p.parts or
        "gh-pages" in p.parts):
    ...

All flake8 checks now pass. No security vulnerabilities detected by CodeQL.

Original prompt

Why is there an error and fix it find the fix and provide the solution for the commit and PR

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[solveforceapp]

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