Add index URL fields and strict coordinate listing mode

[vjovanov]

What does this PR do?

• Adds source-code-url and test-code-url as optional fields in metadata index entries via a new schema file: metadata-library-index-schema-v2.1.0.json.
• Updates index validation to use schema v2.1.0.
• Propagates the new fields through index entry model and index-updating tasks (ScaffoldTask, TestedVersionUpdaterTask, MetadataGenerationUtils) so existing values are preserved.
• Adds -PstrictCoordinates=true support to listCoordinates so it resolves coordinates through getMatchingCoordinatesStrict (including fractional k/n handling).
• Updates contributor/developer docs and adds a strict-matching regression test in TckExtensionTests.

Code sections where the PR accesses files, network, docker or some external service

• N/A (this change does not introduce new external service access)

Testing

• ./gradlew :tck-build-logic:test --tests org.graalvm.internal.tck.harness.TckExtensionTests --stacktrace

Checklist before merging

• I have considered including reachability metadata directly in the library or the framework (see our contributing document)
• I am the original author of all content provided in the pull request, and I did not copy the content from any other source (see the tests section from our contributing document)
• For all tests where I am not the sole author, I have added a comment that proves I may publish them under the specified license (see the tests section from our contributing document)
• I have properly formatted metadata files (see our formatting guide)
• I have added thorough tests (see the tests section from our contributing document)
• I have filled all places where my pull request accesses files, network, docker, or any other external service (see the section above)

[vjovanov]

This should be called PopulateArtifactURLsTaskTests.