Optimize REST calls for single-page virtual directory listings

[syeleti-msft]

Summary

Optimize Block Blob directory listings by avoiding per-directory marker lookups when the hierarchical listing completes in a single page.

• Detect a single-page listing when both the incoming and outgoing continuation markers are empty.
• Create attributes for markerless virtual directories directly from BlobPrefix entries.
• Preserve the existing dirList marker-deduplication logic.
• Continue using REST marker checks for paginated listings, where the marker blob may appear on another page.
• Add unit tests and update the changelog.

Motivation

Previously, listing a directory containing multiple markerless virtual directories triggered one GetProperties request for every BlobPrefix.

For example, 200 markerless virtual directories could require:

1 List request + 200 GetProperties requests

[Copilot]

Pull request overview

This PR optimizes Block Blob virtual directory listings by eliminating per-directory REST GetProperties marker checks when a hierarchical listing is confirmed to be single-page (no incoming marker and no outgoing continuation marker). This reduces REST call volume for directories containing many markerless virtual directories while preserving the existing paginated-listing correctness logic.

Changes:

• Detect single-page hierarchical listings via a new isSinglePageListing(marker, nextMarker) helper and pass the result into processBlobPrefixes.
• For single-page listings, create directory attributes directly from BlobPrefix entries instead of performing per-prefix marker lookups.
• Add unit tests for isSinglePageListing and for skipping marker lookups on single-page listings; update the changelog.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
component/azstorage/block_blob.go	Adds single-page detection and uses it to skip per-prefix marker REST checks when safe.
component/azstorage/block_blob_test.go	Adds unit tests for the single-page logic and for skipping marker lookups.
CHANGELOG.md	Notes the REST call reduction optimization in the unreleased changelog.