fix(models): default CrawlResult links and media to the scraped shape - #2289

Summary

Fixes #2286

CrawlResult.links and .media defaulted to {}, so every result built without them (the robots.txt refusal, "All proxies failed", the exception path in arun, the anti-bot raw-HTML fallback) had no internal / external / images keys. They now default to the keys a scraped page already gets: {"internal": [], "external": []} and {"images": [], "videos": [], "audios": []} (arun moves media's tables to CrawlResult.tables on the success path, so it is not part of the shape). Changing the default covers every construction site, including ones added later, instead of patching four CrawlResult(...) calls.

Behaviour change to be aware of: a failed result's links is now {"internal": [], "external": []} rather than {}, so if result.links: is truthy for it. Iterating over it is unaffected.

List of files changed and why

• crawl4ai/models.py: default_factory for links and media.
• tests/unit/test_crawl_result_default_shape.py (new): a result built without links/media has the scraped shape; the default is not shared between instances; model_dump() keeps the keys; explicit values are kept.
• docs/md_v2/api/crawl-result.md, docs/md_v2/core/crawler-result.md, and the CrawlResult sketch in /execute_js's docstring (deploy/docker/server.py): show the new defaults.

How Has This Been Tested?

pytest tests/unit/test_crawl_result_default_shape.py
4 passed        (3 of the 4 fail on develop)

The offline suites (deploy/docker/tests without the live-server test_1–test_7 scripts, tests/unit, the pool tests) show no new failures against develop.

End to end, on an image built from develop @ 1f68e5b: /crawl with check_robots_txt: true and two URLs of a local test site, so CRAWL4AI_ALLOW_INTERNAL_URLS=true, one disallowed by its robots.txt. Before, and with this branch's models.py mounted over the installed package:

# develop
[shape] HTTP 200
  /index.html            success=True status=200 links=['external', 'internal'] media=['audios', 'images', 'videos'] error=''
  /private/index.html    success=False status=403 links={} media={} error='Access denied by robots.txt'

# this branch
[shape] HTTP 200
  /index.html            success=True status=200 links=['external', 'internal'] media=['audios', 'images', 'videos'] error=''
  /private/index.html    success=False status=403 links={"internal": [], "external": []} media={"images": [], "videos": [], "audios": []} error='Access denied by robots.txt'

cc @nightcityblade @ntohidi

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added/updated unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes