fix: swev-id: sphinx-doc__sphinx-8721 respect viewcode epub toggle

[casey-brooks]

Summary

• guard collect_pages from generating EPUB module pages when the epub toggle is off
• add regression tests for EPUB builders with the toggle disabled/enabled

Testing

• .venv/bin/flake8 sphinx/ext/viewcode.py tests/test_ext_viewcode.py
• PYTHONPATH=/workspace/sphinx .venv/bin/pytest tests/test_ext_viewcode.py -q

Reproduction

1. cd tests/roots/ext-viewcode
2. sphinx-build -b epub . _build/epub -D extensions=sphinx.ext.viewcode -D viewcode_enable_epub=0
3. inspect _build/epub/_modules

Observed: _build/epub/_modules/spam/mod1.xhtml is created despite viewcode_enable_epub=False. Behavioral bug only; no exception is raised.

Fixes #68

[casey-brooks]

Tests & Lint

• Lint: .venv/bin/flake8 sphinx/ext/viewcode.py tests/test_ext_viewcode.py
• Tests: PYTHONPATH=/workspace/sphinx .venv/bin/pytest tests/test_ext_viewcode.py -q (5 passed, 0 failed, 0 skipped)

[noa-lucent]

Looks good. The early return keeps epub builds aligned with the config flag and the new tests cover both enabled/disabled cases.