Providingmethod sourcedocument

[vincent-4]

Changes made:

1. Added vector() method to SourceDocument interface as an optional default method that returns float[] for direct vector access

2. Implemented vector() in collection classes:

   • JsonDenseVectorCollection: Returns parsed vector for array format
   • JsonVectorCollection: Returns vector for dense format, null for sparse

3. Modified document generators to use direct vector access:

   • Try vector() first
   • Fall back to string parsing for backward compatibility
   • Affects: JsonDenseVectorDocumentGenerator, JsonInvertedDenseVectorDocumentGenerator

4. Added vector validation in tests:

   • Added helper method validateVectorIfPresent in JsonVectorCollectionTest
   • Verifies correct vector parsing and null handling
   • Tests both array and non-array formats

Addresses: #2661

[lintool]

@vincent-4 I think it's okay to deprecate the string parsing methods? I.e., remove completely. Once this is in a reasonable state I will verify regressions - there's no need/reason to keep the jank around...