test: cover single-page repository responses

[LuBoys]

Pull Request Check List

Relates-to: #3155

SinglePageRepository tests previously used a subclass that overrides _get_page, so the production implementation was not exercised.

This PR adds coverage for:

• fetching and parsing the repository's single page;
• raising PackageNotFoundError when the page returns a 404 response.

No production code or user-facing behavior is changed.

Testing

• python -m pytest tests/repositories/test_single_page_repository.py --cov=poetry.repositories.single_page_repository --cov-report=term-missing -q — 5 passed, 100% coverage
• python -m pytest tests/repositories --no-cov -q — 337 passed
• python -m pytest --no-cov -q — 3175 passed, 27 skipped
• pre-commit run --files tests/repositories/test_single_page_repository.py — passed
• mypy tests/repositories/test_single_page_repository.py — passed

AI assistance

OpenAI Codex assisted with identifying the coverage gap, drafting the tests, and running validation. The contributor reviewed the complete diff and test results.

• Added tests for changed code.
• Updated documentation for changed code. Not applicable: this is a test-only change.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

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