{Storage} Strengthen blob download path containment tests

[YangAn-microsoft]

Related command

az storage blob download-batch

Description

Strengthens regression coverage for the path-containment fix from #34126:

• verifies the full batch is validated before creating a directory for an earlier valid blob;
• explicitly covers generic single-letter colon prefixes such as a:record.txt, which Windows interprets as drive-relative paths.

This is a test-only follow-up and does not change command behavior.

Testing Guide

• AST syntax check: passed
• Direct Flake8 check for the changed test: passed
• Windows ntpath drive-relative assertion: passed
• git diff --check: passed
• Azure CLI reviewer: approved with no findings

Full azdev test/style/CLI-lint execution could not start in the local environment because the installed Python environments do not contain azure.cli.core.

History Notes

None. This is a test-only follow-up to #34126.

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[Yong Zhang (yonzhan)]

Storage