[STACKED on #932] fix(gateway): accept app-info peer identities

[kvinwang]

STACKED PR — merge #932 first.

Problem

Peer synchronization required a legacy identity form and rejected valid peers that identify themselves using app-info records. Healthy peers therefore failed authentication during mixed-version synchronization.

Root cause and fix

Accept and verify the supported app-info peer identity representation while retaining the existing trust checks.

Implementation

The branch records the following focused implementation work:

• fix(gateway): accept app-info peer identities
• fix(gateway): decode app-info peer identity

Changed paths:

• dstack/gateway/src/web_routes/wavekv_sync.rs

Scope

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

Dependency and merge order

• Parent PR: fix(gateway): validate peer synchronization URLs #932
• GitHub base branch: codex/fix-gateway-peer-sync-urls
• Merge the parent first; this PR contains only the additional delta above that parent.

Verification

• git diff --check origin/codex/fix-gateway-peer-sync-urls..origin/codex/fix-gateway-app-info-peer-identity: 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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