Clarify WriteAsync contract: move document from IngestionChunk to WriteAsync parameter

[Copilot]

WriteAsync implicitly assumed all chunks belong to a single document (pre-existing keys only fetched once for the first chunk's document). This made the contract ambiguous and error-prone. Make it explicit by moving the document association from per-chunk to the method signature.

API changes

• IngestionChunk<T>: Remove Document property and constructor parameter
• IngestionChunkWriter<T>.WriteAsync: Add IngestionDocument document parameter

// Before
public abstract Task WriteAsync(IAsyncEnumerable<IngestionChunk<T>> chunks, CancellationToken cancellationToken = default);

// After
public abstract Task WriteAsync(IAsyncEnumerable<IngestionChunk<T>> chunks, IngestionDocument document, CancellationToken cancellationToken = default);

Internal cleanup

• VectorStoreWriter uses the explicit document param instead of chunk.Document
• IngestionPipeline.IngestAsync passes its document to WriteAsync
• Chunkers (ElementsChunker, HeaderChunker, SectionChunker, SemanticSimilarityChunker, DocumentTokenChunker) drop the now-unnecessary document parameter from internal helpers and chunk construction

fixes #6970

[adamsitnik]

It's not as bad as I thought it would be.

@roji could you PTAL and let me know if it addresses your feedback from #6970?