fix: fall back to httpx when playwright is missing

[MobiusL00p]

Summary

• catch ModuleNotFoundError when the /web scrape path tries to use Playwright without the optional dependency installed
• mark Playwright unavailable for the current scraper instance and fall back to the existing httpx scraping path
• add a regression test covering the fallback behavior without requiring Playwright to be installed locally

Verification

• python -m pytest tests/scrape/test_scrape.py -k playwright_missing
• python -m pytest tests/scrape/test_scrape.py -k "playwright_missing or scrape_text_plain or scrape_text_html"
• python -m compileall aider/scrape.py tests/scrape/test_scrape.py

Closes #4911.