Use maps.Equal for full metadata staleness detection in checkSession instead of per-key comparison

[yrobla]

Context

Raised as a suggestion on the session manager's checkSession implementation.

Currently, the cache liveness check in checkSession compares only MetadataKeyBackendIDs to detect cross-pod metadata drift:

if cachedIDs, present := sess.GetMetadata()[vmcpsession.MetadataKeyBackendIDs]; present {
    if cachedIDs != metadata[vmcpsession.MetadataKeyBackendIDs] {
        return cache.ErrExpired
    }
}

Problem

This approach requires checkSession to know which specific metadata key to inspect. If new metadata keys are introduced in the future, staleness detection won't cover them unless checkSession is explicitly updated. The session manager doesn't need to care about which key drifted — if in-memory state doesn't match storage, it should evict and let the next restore pick up current state.

Suggestion

Replace the per-key comparison with maps.Equal over the full metadata maps:

if !maps.Equal(sess.GetMetadata(), metadata) {
    return cache.ErrExpired
}

Both sides are map[string]string, so maps.Equal works directly with no reflection overhead. This gives all present and future metadata keys staleness detection for free without any further changes to checkSession.

Acceptance Criteria

• checkSession uses maps.Equal to compare the full metadata maps
• The per-key MetadataKeyBackendIDs comparison is removed
• Existing tests continue to pass; no new metadata-key-specific logic needed in checkSession

References

• PR Add LRU capacity to ValidatingCache, remove sentinel pattern, add storage Update #4669 where the per-key comparison was introduced