[STACKED on #891] fix(guest): make KMS failover ordering deterministic

[kvinwang]

STACKED PR — merge #891 first.

Problem

KMS endpoint failover depended on map/inventory iteration order. Different runs could choose different first endpoints, causing nondeterministic latency and making recovery behavior change after restart.

Root cause and fix

Sort/retain an explicit endpoint order and apply failover in that deterministic sequence.

Implementation

The branch records the following focused implementation work:

• fix(guest): make KMS failover ordering testable
• style(guest): format KMS failover tests

Changed paths:

• dstack/dstack-util/src/system_setup.rs

Scope

This PR addresses one logical guest finding. It intentionally excludes the acceptance-test infrastructure from #841 and unrelated product fixes from #840.

Dependency and merge order

• Parent PR: fix(guest): safely write gateway private state #891
• GitHub base branch: codex/fix-guest-gateway-private-state
• Merge the parent first; this PR contains only the additional delta above that parent.

Verification

• git diff --check origin/codex/fix-guest-gateway-private-state..origin/codex/fix-guest-kms-failover-order: passed.
• The declared base was verified as an ancestor of the PR head.
• Focused compile/check verification was run for the changed component where applicable; non-Rust packaging or configuration changes were reviewed against their exact branch delta.

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[kvinwang]

Closing because the existing implementation already performs deterministic, sequential failover in configured URL order. This change primarily refactors the loop for testing and does not address the claimed inventory-ordering root cause. URL normalization, if needed, should be proposed separately as a focused change.