Add logging, Timestamp warning, and Cosmos DB limitation docs

[philnach]

Closes #239

Problem

When using QueryFilter with Timestamp on a Cosmos DB Table API source, the DMT silently returns 0 results with no indication of why. Per Microsoft documentation Query by using an OData filter, "Azure Cosmos DB blocks the queries on timestamp properties." This is a known Cosmos DB limitation, not a DMT bug.

Changes

AzureTableAPIDataSourceExtension.cs

• Log table name and QueryFilter value at read start
• Log item count after enumeration
• Warn when 0 items are returned
• Detect Timestamp in QueryFilter and emit a specific warning about the Cosmos DB limitation
• Replace manual GetAsyncEnumerator()/MoveNextAsync() with await foreach with proper WithCancellation(cancellationToken)

README.md

• Add prominent callout about Cosmos DB Table API Timestamp filter limitation with links to official docs and SDK issues
• Add workaround guidance (use custom datetime properties instead)

Testing

• All 31 existing unit tests pass
• Verified against a live Cosmos DB Table API account: confirmed all Timestamp filters return 0 rows while non-Timestamp filters (PartitionKey, custom properties) return correct results.

[philnach]

@markjbrown / @bowencode, whenever you get a moment if you can code review and approve the PR it'll help bring clarity to the Query with timestamp functionality in DMT.