Draft: <fix>[zbs]: ZSTAC-86782 backport MDS failover

[ZStack-Robot]

Summary

Backport the complete ZSTAC-80595 fix for CBD/ZBS volume-clients anti-split-brain checks. When the selected MDS is unreachable, the request now uses the existing next-MDS path instead of ending before failover can run.

Source provenance

• zstack !9153 / 3b5bda3b76aef968a911d18e35b3b30bd0cab803: use the synchronous MDS caller with a 30-second timeout and enable tryNext.
• zstack !10320 / 1f65e4e5a8a547176cab7c8ed029e3288765d9f6: convert synchronous REST failures into failed agent responses and add the regression case.

Changes

• Keep /zbs/primarystorage/volume/clients timeout at 30 seconds and enable next-MDS handling.
• Let synchronous network failures reach the existing failed-response branch.
• Backport the existing ZbsPrimaryStorageCase regression scenario; the only cherry-pick conflict was target-branch import context.

Testing

• git diff --check upstream/5.4.12..HEAD
• Source and backport production patch-id match for the final sync-failure change.
• Remote x86 plugin/zbs package
• Remote x86 ZbsPrimaryStorageCase

Resolves: ZSTAC-86782

sync from gitlab !10547

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📒 Files selected for processing (3)

• plugin/zbs/src/main/java/org/zstack/storage/zbs/ZbsMdsBase.java
• plugin/zbs/src/main/java/org/zstack/storage/zbs/ZbsStorageController.java
• test/src/test/groovy/org/zstack/test/integration/storage/primary/addon/zbs/ZbsPrimaryStorageCase.groovy

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