Return BlobNotFound for conditional reads of missing blobs

[Copilot]

Azurite returned 412 ConditionNotMet for GET/download and HEAD/get-properties requests against nonexistent blobs when If-Match was set. Azure returns 404 BlobNotFound before evaluating target conditional headers.

• Blob metadata stores
  • Updated Loki and SQL download/get-properties paths to check missing or logically uncommitted blobs before validateReadConditions.
  • Preserved conditional validation ordering for existing blobs.

if (!doc) {
  throw StorageErrorFactory.getBlobNotFound(context.contextId);
}

validateReadConditions(context, modifiedAccessConditions, doc);

• Regression coverage

  • Added focused @loki @sql tests for GET/download and HEAD/get-properties with If-Match targeting a missing blob.

• Release notes

  • Added an Upcoming Release Blob changelog entry for the Azure-compatible error precedence change.

Original prompt

Fix Azure/Azurite issue #2751. For GET/download and HEAD/get-properties requests targeting a nonexistent blob with If-Match, return BlobNotFound/404 before evaluating target conditional headers, matching Azure instead of returning 412. Preserve conditional behavior for existing resources and unrelated operations. Apply consistently to Loki and SQL metadata paths and add focused GET and HEAD regression tests tagged for both backends. Add Upcoming Release ChangeLog entry and run targeted tests, build, lint, and diff check. The locally validated version changed LokiBlobMetadataStore.ts, SqlBlobMetadataStore.ts, tests/blob/apis/blob.test.ts, and ChangeLog.md. Include 'Fixes #2751' in the PR body.

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🟢 Approval recommended

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