Fix cross-link validation

[reakaleek]

Context

The build did not error in elastic/elasticsearch@4b0dfd1#r2398182955.

Reported by @leemthompo

Changes

• Remove the resolving logic based on "declared repos".
• Add test

[reakaleek]

@theletterf I can see the code part that I just removed was introduced in https://github.com/elastic/docs-builder/pull/1615/files#diff-2ae5dfa0b71419475ba9627d282033845c278d67ed966bdfd37ad40b0b0133aaR93-R101

I can see the comment, but can you elaborate on the purpose?

[Mpdreamz]

LGTM, we have to chase and fix few bad links first before pulling.

[Mpdreamz]

content source current and edge:

Error: 'troubleshoot/observability/apm-agent-dotnet/apm-dotnet-agent.md' is not a valid link in the 'docs-content' cross link index: https://elastic-docs-link-index.s3.us-east-2.amazonaws.com/elastic/docs-content/main/links.json
NOTE: /Users/mpdreamz/Projects/docs-builder/.artifacts/checkouts/current/apm-agent-dotnet/docs/docset.yml

content source staging:

Error: 'troubleshoot/observability/apm-agent-dotnet/apm-dotnet-agent.md' is not a valid link in the 'docs-content' cross link index: https://elastic-docs-link-index.s3.us-east-2.amazonaws.com/elastic/docs-content/main/links.json
NOTE: /Users/mpdreamz/Projects/docs-builder/.artifacts/checkouts/next/apm-agent-dotnet/docs/docset.yml

Error: 'reference/edot-android/supported-technologies.md' is not a valid link in the 'apm-agent-android' cross link index: https://elastic-docs-link-index.s3.us-east-2.amazonaws.com/elastic/apm-agent-android/main/links.json

Error: 'reference/elasticsearch/mapping-reference/semantic-tex.md' is not a valid link in the 'elasticsearch' cross link index: https://elastic-docs-link-index.s3.us-east-2.amazonaws.com/elastic/elasticsearch/main/links.json

[Mpdreamz]

@theletterf @leemthompo Can you help us address these observability and elasticsearch bad links?

[theletterf]

Sorry, I didn't catch that block during LLM generation. It smells like the typical "let's make this simple" way of Claude of fixing things. Since local builds were catching bad crosslinks and tests were in place, I didn't detect this. My bad.