fix(guest): isolate local key providers from KMS inventory

[kvinwang]

STACKED PR — merge #946 first.

Problem

Local key-provider entries were mixed into the remote KMS inventory and passed through KMS failover logic. A local provider could be treated as an unreachable KMS endpoint or reorder remote selection.

Root cause and fix

Keep local providers in their own path and build the failover inventory only from actual KMS endpoints.

Implementation

The branch records the following focused implementation work:

• fix(guest): isolate local key providers from KMS inventory
• style(guest): format provider route test

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: [STACKED on #891] fix(guest): make KMS failover ordering deterministic #946
• GitHub base branch: codex/fix-guest-kms-failover-order
• Merge the parent first; this PR contains only the additional delta above that parent.

Verification

• git diff --check origin/codex/fix-guest-kms-failover-order..origin/codex/fix-guest-local-provider-inventory: 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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